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wiui loos,.- eai'iu. A lu-noi-ae po.ver euglne, veighlu^
eight cons, ran with four waggoasntuictK-d Co It out to
a colliery 12 miles from Edinburgh; thero received a
lo.ill amounting, with tbe four waggons, toil.! tons,
making the weight of the whole In »ons -, unit thou
returned up inclines of 1 In lis to Kdlnburgh. It wended
1U way, with its trnln of 90 feet, with perfect facility
through the n*rr«*w streets of the old town, whicli
chanced on the occasion to be thronged with vehicles
carrying people to somenpeh-nlr festivity. It turned
all manner of sharp corners, rundown the Btcephil! to
Leitb, entered ii lane, nud drove In through the gntoB
of the factory, where it delivered Its load. An eufrlno
was driven into n newly dug potato h>ld, and there
ran about In every direction, leaving die soil quite
undisturbed. A 10-horse power engine, drawing a load
of 1? 'ons of pig-iron, wa< driven along the Grauton
road at the rate of 8 miles an hour. The same euglne
was run along the sea ssnds from Portobello to Joppa,
running through the loose, dry sand, over the soft,
wet sand, and even through a creek of running water,
with as mnch ease as If it hvl been driving along
a turnpike road. It Was driven over long beds
of broken Hint laid down for road mending, and tbb
motion to those riding on it was as smooth aud plea-
sant as If it had been going over a lawn, while the
stones remaiuel quite undepressed. A six-horse
power engine, weighing 0 ton*, look a load of 31 tons
up an Incline of Hi 18. Engines with omnibuses at-
tached to them have run frequently through Edin-
burgh up the long steep hill fr.m Ltith, ttnd up and
down tho steepest streets of the city, always without a
break. Their speed Is from eight to ten miles an hour,
and sorno very handsome stenm omnibuses are now
being const' ucted on this principle. In Lelth the
road steamer Is constantly BrnfJlbyed In conveying
marine boilers {weighing 2S tonsi.-tnarlnc screws, rail-
way locomotives, and other enormous pleeeB of ma-
chinery to thtttoclcs or tho railway stations. When
harnesses] to Its loud. It rs driven straight to Its desti-
nation without a single stoppage, irbd without Inter-
fering In any way With the street traffic. The roatl
steamer Is likewise found to answer admirably in
etieet-rolllsg, drawing tile roller, which is Beparate,
behind lc. A rind steamer df six-horse power, Whldli
belougs to the owner of some very extensive flour-
mills, has been running for seven mouths between
Aberdeen and the mills. The distance each way is
three miles,and this It accomplishes in an hour. At
first It performed four trips a day, but latterly It has
been making six, taking a load of ten tons each time.
Tbe road over which It travels is perhaps the worst
bit of road in the kingdom, being narrow and tor-
tuous, and tho gradients for one-half of It varying
from 1 In V to 1 In 8. Up to this Incline of 1 in 8 the
road steamer, which weighs six tons, takes In tow a
load of ten tons. It may help to realise what 1 In 8
represents, to mention that tbe steepest gradient In
crossing the Slmplou Is I tn 13. This engine has al-
ready run over 2500 miles, aud has cartled nearly
£000 tons, running six times dally through tbe chief
streets of a busy city. To prove how iuuapable tbe
a tenner is of Injuring the road, various substances,
such as bits of coal, potatoes, carrots, Ac., have been
thrown in lis path, and after it had passed over them
they have been picked up uncrushed. The India-rubber
tires nave often been compared to the elephant's foot,
with its soft and yielding pad. The road steamer is
exceedingly trim and compact. It runs on three
wheels, two large ones, and a smaller one In front.
The india-rubber tires for the three wheels of a
ten-horse power enirlne weigh 14 cwt. To cast Buch
enormous masses of vulcanised tndl i-rubber was in
Itself a question demanding no little ingenuity, study,
and enterprise. The tires are guarded by flexible
shields, lormed of open steel bars, which give an ex-
cellent " bite " or hold, upon tbe ground, and while
tbey do not In any way. Interfere with the elastic

?lay of.tlie indiarubbcr, they afford such protection to
C as to render it virtually lndestrnucible. Tbe
shields, which are removable, are not used for driving
over lee or frozen snow, as on such surfaces Iron will
not bite, and here the India-rubber Is of Immense ad-
vantage, as It runs over them with perfect ease, and
without slipping. In running through sand, also, as
in Egypt, tbe shields are entirely dispensed with.
These engines are now being built for tbe most vari-
ous purposes, both for home and foreign use. and are
being sent to the remotest localities. Till within the
last few months, the advantages of tbe road steamer
had been regarded as consisting en 11 rely in its carrying
powers, but during the past, summer a new field
of action opened out to it, which eminently enhances
Its value. The judges of the Royal Highland and
Agricultural Society, held onTuesday, August 17, on a
farm at Llberton, near Edinburgh, a trial of the
ploughs, reaping and mowing machines, exhibited by
them at their last show, and In their presence, and
that of a large assemblage of farmers and engineers,
tbe road steamer accomplished what has been so much
desired, but had hitherto been considered unattain-
able—ploughing by direct traetlon. With two double
furrowed ploughs attached to it, it commenced its ope-
rations, and without a single hitch, dirficultv, or im-
pediment, drew four wide, deep, even furrows. It
went straight from one end of the field to the other,
-then turned far more easily aud iu less space than the
horses were doing, and ploughed its way back again,
having on its return Journey to plough up a hill with
gradients 1 in 12. it was a six-horBC power engine,
but its scrength was greatly iu excess of its work, so
that It was requisite to keep the furnace door open,
and it was evident it would have drawn six furrows
instead of four. Occasionally Its progress was too
rapid to suit the convenience of the ploughman, but a
single "woa" from bini checked it instantaneously. It
was pleisant to contrast the smoothness and ease
with which the engine performed Irs task, -Aith the
desperate struggles of the horses ploughing along-
side it, through the stiff, heavy Boll. Tbe work done
by the rond steamer was ss perfect as work could be,
tbe ploughs being set ns deep as possible, and the fur-
rows entirely true and accurate. The surprisingcapa-
bilities of the road steamer are due to the face that
owing to the Indla-rubbor tires, thewhoels do not sink
in the least degreeon the softest grass land, and at
tbe same time tbey bite the surface wlthBnch extreme
tenacity, that not tho slightest slip was vlaiblo, even

when passing up tho steepest incline. The engine
runs in front of the ploughs on tho unplou^hed litud,
whsreby nil possibility of compressiug or poaching
the soil nftor it is turned over Is avoided, while on the
unploughed land no Indentation whatever is made, as
tin? Bott india-rubber cushions preserve it from all
pressure. When the road steamer had completed its
Work, all present expressed the greatest gratification,
nud the benefits to be derived by farmers from this
new Implement were eagerly pointed out. Ploughing
by stesm has hitherto been so costly a process, that
only very large fanners could employ It. The road
steamer does not cost n third of tho price of the cum-
brous engines at present In use, and it, of course,
does awny, likewise, with the constantly recurring ex-
pense of repairing Wire ropes and tackle. At first sight
It might appear that more power Woi Id be consumed
by a travelling engine th«b by an engine 'Which stands
In one spot, and works by means of a rope This, how-
ever, is not the case, for the friction of the Wire rope
and the gear aud tackle connected With it consumes
more power than is needed to propel the road steamer
backwards and forwards over the field. This will be
very clear to railway engineers, Who know how far
more expensive It Is to pull a train lip an incline by
means of a fixed engine and rope, than to draw it up
even stiff gradients by a locomotive. But the chief
merit of the road steamer to farmers will be that it
Will allow them to dispense very greexty With horses,
which the so-called portable eJnerftes flo not permit
them to do. It is ns much OB the portable engines
cau do to move themselves, and even their fuel
anfi -Water have to be carried for them. The road
steamer, on the contrary runs With the greatest
ease over anv kind of land, Cuthb in loirs space than
horses, and fetches its own fuel and water. Besides
ploughing, it can perform every dther farming
operation—It Ban drive the thrashing machine, draw
the reaping 4shd mowing maehlucv, bring manure to
Hie fields, and cart thetfrarn to tho market, so that,
by its adaptability'to these various purposes. It will
enable farmers to reduce rtielr staff or horses to a
minimum, which In these days of dear provender, will
be counted no light gain.

strong; but he alone advises "copper" wire; and likewise says, use a "smo/nunip of zinc.*'

Now. surely all these recipes will not grow a " zinc plant" (!) iu perfection. I always imagined the rationale of this "pleasing ornament " (as the man in the streets says), was that the acetic acid had a greater liking for the zinc than the lead, and so the lead was deposited iu flake-like chains suspended from tbe zinc. So, of course, the smaller the zinc, so much tbe less the growth of the plant; aud the weaker the "solution," the smaller the quantity of lead to be deposited ; for when the zinc is all appropriated tha excess of acetic acid remains with the lead, and, per contra. If the " solution " is exhausted before the ziae Is "dead," it is obvious the tree will not be the size it might have been. If my notion is wrong, I will thank our friend " Sigma" to set us right.

Saul RritxA.


Sir,—I was in hopes that some of your qualified correspondents would have answered "Veritas'" queries about the eccentricity of the earth'b orbit. Ac. Tho -005908 is the fraction between tbe two distances he gives as the mean and aphelion (p. 520, Vol. X.). The eccentricity is equal to the difference between the mean distance and tho perihelion divided by the mean distance, as—


Moan dis. - perih. dis. = = Kccoid.

mean dis. Now the horizontal parall&x is stated by Mom to be 8'-932; this would give a mean distance of—

91 510 000 miles - to log. 7-961.100

Earth's radius vector, Jan. 2 = iu perls. ,, 9 992660

Miles 80 983000 - perihelion distance .. „ 7-93110"

Mean 91*Mn-uODi .,«, _ ,..-,«,, _,«.

Perth. W983 OOuj ",lfl- = !'33 00D miles.

Then dill, divided by mean dis.

Log. of dirt. = 0-185542

„ mean = - 7-9W500

001875 = eccentricity 2^24012

R206264'-8 X 3983


Sib,—Finding so many inquiries addressed to me by various correspondents, I feel it incumbent upon me to endeavour to reply, although still exceedingly unwell in health. I shouldadvise jrour correspondent. S. Samuel, No. 2019, page 611, No. 259, to make the series of experimental pipes, as what I have commenced and purpose doing uffder the Section "Pipe Construction" is to place before the amateur the various plans, leaving the selection to the taste of the Individual. If the following arrangemsnt will be of use, however, as a guide—being something like the arrangement I should use myself—It is much at your correspondent's service: Swell. Open diapason, principal, hautboy, great, stop diapason, dulciana, flute, fifteenth.

In reply to "M. P. 8 ," No. 2082, page 613, I should advise him to use 60 valves, such as I have described, not Increasing the size, as If that is done the angular motion and consequent loss of wind Increases at a disproportionate ratio.

Whilst thanking your correspondent, "C. R. O., Hants," on page 057, for his courtesy, may I venture to inquire if the idea bas never stru-k htm of placing the 12 bourdon pedal pipes iu fuw rows, the five largest at the back and the remaining seven In front of them? This will answer well, and us he states that depth Is not so great an object, I presume it Will meet his requirements.

In reply to "W. D.," No. 2154, on phge 21 of the last number, be will find the description and figures of the bellows' valves he mentions In my communication on page 212, No. 242 -, the other Inquiries he makes are not in my department, but no doubt some of the skilful writers who grace your columns with their contributions upon the harmonium will afford the requisite information.

I presume your correspondent, "Organine," No. 2190, page 22, is a new subscriber, from the nature of his Inquiry. I beg, In reply, to refer him to page MO. No. 223, where he will find I havo given, I trust, a full description of what ho asks for. Am Adept.

Sir,—With regard to the manganese battery, I may
state that I use a saturated solution of common Bait
in tho outer Jar; for what Is the difference between a
saturated solution of chloride of ammonium and a
saturated solution of chloride of sodium? Tho differ-
ence is this: chloride of ammonium Is Is. per lb., and
chloride of sodium Id. for 31b. ; and I Hud that chloride
of sodlnm is equal to chloride of ammnrJintn, and both
theso solutions are greatly improved by tbe addition
of 3 drops of common hydrochloric add to the quart of
solution. I can say that one quart cell has been in
action ten months with a saturated solution of common
salt. 1 havo also used those batteries for electro-
plating, and can therefore recommend them to anyono
who wishes a clean, cheap, and efficient battery for all
purposes where a constant battery is required. Query:
Why should this battery not answer for the electric
light? Six quart cells, when connected, intensify
fusion and give it powerful flash. The mixture I use
In the porous cell Is lib. of pounded gas carbon and
31b. of black oxide of manganese. This mixture
must be moistened with tbe saline solution when the
battery is first made. A. J. Jarman, liamsgatu

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= 5SW12S0 The 3963 miles is the latest determination of the equatorial semi-diameter by Capt. Clarke, of the Ordnance Survey. 8. B.

ON THE METHOD OF LEAST SQUARES. 8iR,—The objoot proposed in the solution of Equations, however formed, Is the determination of the value of some unknown quantity involved in tbe equation, by certain operations with given numbers and accompanied with appropriate symbols which direot theso operations. In tbe case of one unknown quantity, affected by co-efficients absolutely known, tbe determination of the uuknown, in terms of the known quantities, Is a problem that offers little difficulty, and provided that the number of equations Is equal to the number of the unknown quantities, the discovery of tbe unknowns Is a simple algebraical operation. But when the unknown quantities are affected by eo-efneients only approximately kuown, the resultiug value of the term sought will be affected by-a certain error. To thosewho have not advanced Into tbe higher branches of mathematics, it may seem probable that if a certain result absolutely satisfies an equation, then a quantity very nearly equal to that result will very nearly satlsfv the equation, nearly being understood in the same manner In both cases. But this does not necessarily follow, and we are led to ask what is the test of approximate equality. Let us take the equa

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8ra,—I have been much amused by reading In our Improved journal sundry recipes for the production of what I have always known as a lead tree, although It now appears under a misnomer—zinc plant. "Ernest'' says, use a "feeble" solution of lead; "A. A. A." says, put a " little " sugar of lead In—meaning after the sine and wire are arranged ; " Beta "say>. "granulated sine In a Mrong solution; Henry Chapman does net say whether the solution should be "feeble" or

tlon there is absolute equality. Now,
the word nearly any signification we please. Let tie
suppose that an error of 0-0001 Is a quantity so small
that It can be neglected, and that the result of our so-
lution of tbe equation has given us r = 0O0009 and a
= 0-00001, x — a then gives us 0-00008, which as It la
less than 00001 can be neglected. Are we entitled to
assume that the equation is approximately solved?

Certainly not. Try the other test, — = 1. Inserting


0-00009 the values of x and a, we have — = 9, which, is

0 00001 not by any means a small quantity. ^^

Now, the method of least squares nssumes that the most probsble values of the unknown quantities, tn any system of equations, are those which make Use sum of the squares of the residual errors a minimum. By the residual error, we mean the value of the equation after the approximate values ol the unknown quantities havo been substituted. This principle may serve as the basis of tbe solution of many otherwise indeterminate problems, and its adoption is justified by the calculus of probabilities, bnt which Is of a coo mathematical nature to be recapitulated hero. Wewillproceed at once to explain tbe method employed lu solving equations of condition, and tofix our Ideas let us suppose that these equations arise from comparisons of observed and tabular places of a planet. If we suppose that the elements of a planet's orbit are not exact but only approximations to tho truth, and if wo suppose that the formula for deducing a place of the planet from these elements is absolutely exact, every observation will differ more or less from the computed place. This

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. which 1* culled the errorof the tables, must necessarily be a function of all the elements eater lot» uhr comr-ar-atiou of the plauet's place, which we will call », tad if we denote the elements of the orbit by x, i!, z, ic, the equation must take the form

* =ei + by + сг, &c.

Another observation will gire

e? a! x + Vy + & *,&e.

e* = o' i + V У + С z, Ac аша there will be aa шллу équations as there are ob»«TTailon». Now. i hi и: can be at tbe moat. In a problem similar, to that that we are considering, seven unknown, tad therefore neveu observations would be •ufErietu tu deteruiluc thü corrections to the elements. Bat esichohscrvatioii I* effected bj an error, or rather numeroa» error«, arising fruin accidentai causes, anil whi<-b cannot be subjected to computation, such as tremor* of tbe instrument from wind,errors in refraotlon produced by changes la the temperature of the atmosphere, which wilialsopioducs anomalous changes In tbe part" of an instrument, and above all by imperfection of tbe senses, and an incapacity In the observer to estimate very small intervals of time and space. Thf on¡Y meljapd ol removing these errors is. to multiply tbe number of observations, when it U assumed that aü small accidental errors will destroy one another. But to increase the number ol observation« ia to increase tbe »umber of equations of eonditlon, and heuca ¿rise» tbe necessity of inventing a method of nolutipn iu which each equation shall equally

Îiartieina&es ад<1 oi defiviwg a result that shall no aa ittle лЯхнА a* nosaible by error, and If possible to determine the avuouut ol probable error in tbe result.

Squaring each of the equations of condition, and taking the sum, we snail have an equation of tbe f ollowi nc form :—

е* + е" + e*» + Ac. = (e* + o^ + o** + Ao ) *' + îis'ii + db' + 4c.) + 2xt (ас + oV + 4c) An equation evidently of the form,

K = Mz3-rNzT + P And if wc are to mate it a minimum, tbe first differential co-efficient must be equal to zero,

. ". K> + N = О


x (a» + »ч + Ac.) + y (o i + а/ Ь' + Ac) + г с

of d Ас.) + &с. = О.

Henee we bare tbe following rule to form the equation that gives a minimum f'»r any one of tbe unknown quantities : Multiply each equation of condition by the coefficient of tbe unknown quantity in that equation, taken witb its proper Bleu, and then add together all these product*; and this being repeated for each unknown quantity, there must result the same number ol équations aa unknown quantities, whence each Is reduced by ordinary algebraical operations.

AVc give as a numerical example the following

equations that are Riven by Gauss, and have been

employed ever since as an example :—

x - y + - S = О 3* + »y-Hi- 5 = О

i x + у + и - а = о

+ Зу + 3*-Ы = 0

If only three equations had been given, a very slight knowledge of elementary algebra would have been sufficient to obtain a correct result, namely:

18 23 13

* = —. » = —. » = —,

7 7 7

And these value» would be final, with no means of judging of their accuracy. But if these values be substituted in the last equation, the second member

S 8 becomes instead of zero. This is termed

7 7

the residual error; and If a result be deduced from any three equations, the fourth will not be satisfied, bat leave a small error. We must use the rule given above for the normal equation of x, multiply each equation by L 3, 4, and - 1, and take the sum, for JT, It will be necessary te multiply each equation by - 1. 2, J. -3. anl add the results similarly for z, we shall have, then.

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The solution by ordinary methods gives,

x = 2 470. y = 3-551. and г = 1-916 If these value» be substituted in tbo original equations, it will be found that no one equation is accurately satisfied, the reelduala being

- 0-2«, - 0-068, + 0095, — 0 000.

To determine the probable error of each unknown * t rather long »nd in trícate computation: we indicate «Tribe method her«, the proof of what is inadmissible. H Cae first normal equation write — 1 for the absolute tera^. Nod Ijq the other equations zero, for each of the absoUte trrmt: the value of x' then found from these equates will be the reciprocal of the weight of the value ul z tound by the general elimination; similarly tor tbe other unknowns. Writing then,


(employing dashes to distinguish the particular results) we shall derive by tbe ordinary methods of elimination,

80 51 41

«* = . У = —, «" = —,

19889 737 2S11

which, being inverted, give the required weights, which we will denote by px, py, and pz. In order to find tbe moon error oi м. y, and z, we must take tbe sum of the serious of tbe residual errors that we have already determined. The squares will be respectively Ouom 0-0 i«. 00090, and о шнв, the sum of which is equal ta0080t, the square root of which is 0281 = с the mean errors ui x,y. and e are given by the for ши 1 be

tx = , similarly for у and », mutatis mutandU.


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t,j - = 0077


и ~ = 0'0»

-/f which are the mean error of x, y, and z. This is a general outline of the method employed in solving equations of condition by the method of least squares. The computation is necessarily long and troublesome, but the acouracy of the result has never been doubted, nor has any other method been suggested that promises by shorter computation to give a result equally correct. Tho late Professor Encke employed this Ingenious process in a variety of ways, and It has now become a most formidable englue in modern astronomy. Omicron.


Sib,—As regards the letter of "J. T." respecting " Violin Varnish." I quite i

page 14, agree with what he says. 'Th.- Information whlch"he desiros would be acceptable to many of your readers. I have myself, at the present moment, a valuable Italian instrument, on which I have plaved for more thin 20 years, but Its belly is without varnish. The cause of this 1« aa follows:— I purchased the Instrument, at a high figure, at the sale of an eminent violinist's effects, and by imagining that another coat of varnish would improve the tone of the old violin, I made and applied a varnish of naphtha and shellac, but the result very much disappointed me, for *he sound was somewhat deadened, although the alluring sweetness of tone remained.

In a few weeks I deccrmlned to scrape off the varnish, and did so carefully with a piece of gloss. Dut unfortunately the first ceat of varnish came off with the second, and tho belly of the violin has, since that time, remained bore, for I have been af.'aid of again meddling with it. I keep the instrument in a silkbag in a mahogany case, and tbus the air does not appear to affect it. Notwithstanding this, I should, of course, be highly pleased if I could restore it to its original appearance.

1 know there are many " Violin Varnishes," but I cannot venture to apply any of them, neither would I place it in the hands of a violin maker until a suitable spirit varnish like the Italian varnish spoken of by M. Grivel). has been tried and approved. It Is strapge, and I have often thought so, that neither Rode nor Louis Spohr, the latterT>f whom has touched upon almost everything connected with the greatest or all instruments, has said anything In reference to the varnish.

There can, however, be no doubt, amoarst violin players that the quality of the varnish affects in a considerable degree the vibration of the instrument, and I am glad that yonr correspondent '■ J. T." has drawn attention to the subject, although I had intended doing so myself. Tautau.

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Sib,—So long ago as October last year, In tbe number for the 22nd or that month, on pace HO, "Reltvot " gave some of the dimensions of his compound engine and heiler, and solicited the opinions of "practical engineers" upon the same. I replied, asking him to Send more particulars, and promising to give him the information he required. He gave the required dimensions in the number for November 19th, on page 242, and I at oace set about writing this lauer, but I laid it by for reasons I need not trouble you with, and did not resume It till now. 1 hope it is not too late to be useful to " Relivot."

With your permission I will just recapitulate bis questions here. Flrat, " Will the boiler supply those engines with steam?" Second, "Are the areas of pistons, and parts, and travel of valves, properly proportioned ?"; and after stating in his last letter, "That those engines are working in connection with a water wheel," aud that " their load is varying from day to day." he asks "Weuld an independent out off valve be advisable?" The dimensions of his boiler are—" Shell 30ft. long, 7ft. diameter, containing two furnace tlues. 'Z' 7" diameter, with grates 7ft. long." This boiler Is capable of evaporating about 50 cubic feet of water per hour, witb good coal and sharp draft, and the steam from that quantity of water If used In a well-constructed condensing engine, cutting off at about one-third of the stroke, will produce about 100 to 120, or perhaps more, indicator horse power.

The stroke of tbe high pressure cylinder is 42ln., or 7ft. for the double stroke, and it makes 60 strokes per minute, or 420ft. per mtnute piston speed, and the diameter ol the cylinder is Ши. The steam port Is Hin. long, and lflii. wide, or 15J square inches, which Is very good proportion. The speed of the low pressure piston is the same as the high pressure one, aud

Its diameter is 22in; the steam port is 15tin. long, and

11 llin. wide, or 22— square Inches area: this I consider

16 rather small, and if dedgnin; such a cylinder with equal puiton speed, I should dive a »team poet of 28 oç 29 square inches area, but such aa it ta it will do very well. There are worse thinga by a great deal about this engine tbao the proportions of its steam ports, as wo shall see by- and-bye.

The steam is cut off from the high pressure cylinder at 31 per cent, of the stroke, or 141tn., and the diameter of the cylinder being 16in., we have a space of 185 cubio feet to he filled twice every stroke, or 3 30 cubie feet per revolution of the crank shaft; tbis at 60 revolutions per minute Is equal to 198 cubic feet, or 1188j> per hour, to which we may add one-fourth for waste in filling ports, pistou clearance, leakage and cooling, and we have 11850 cubic feet of steam required per hour; this at 001b. per square inch above the atmos

Ïhere. or 751b. total pressure above a vacuum, requlrea or its formation 39 cubic feet of water, hut the boiler Is capable of evaporating 50 cubic feet per hoar, se there can be no question of its sufficiency.

We will now take a look at the proportions of the slide valves, and see how they acquit themselves In the performance of their duty of distributing the steam to the cylinders, but before I do so I shall just slate what are the proper proportions and essential conditions of a good Compound steam engine aa far aa its valve gear is concerned.

The valve whioh admits the steam from the boiler to the high pressure oylinder should be capable of cutting off at any point, from, say, 10 to 96 per cent, of the stroke, and of detaining tbe expanding steam in the cylinder to do duty oa the piston to nearly the end of the stroke, and this valve should, if posfrlble, be placed under the direct control of the goveruor.

Tho communication between the high and low pressure cylinders should be free and unrestricted from the beginning to very uear the end of the stroke, in order that no illegitimate or unnecessary baok pressure may exist to retard the motion of the high pressure piston, and that the low pressure piston may receive the full benefit of the unrestricted expansion ol the steam contained between the two pistons.

The communication between the low pressure cylinder and the condenser should also be freely open from tbe beginning to nearly tbe end of the stroke, in order that its piston may continue to receive the benefit of the vacuum as long as possible.

"Relivot's " high pressure valve has rjin. of travel,

3—In, of lap, aud no lead, and the exhaust edges of the/ 16

valve and those of the ports are equidistant -, with these proportions this valve cuts off the eteam at 34, per cent, of the piston's stroke, or ll-25in., and exhausts nt 78 per rent., or 42 75in.. aud at the same instant compression begins on the other side of the platón. The valve of the low pressure cylinder also

13 3

has TJiu. of travel, 2—la. of lap, and —In. of lead, and

1« 16

the exhaast edges of the valve and ports are equidistant; these proportions cause It to cut off the commanicntlon with the high pressure cylinder at 36 per cent., or l.vr.'iii.. and to exhaust at 78 per cent, or 3275tn., exhaustion ceasing aud compression beginning at the same instant on tbe other side of the platón.

Now two serious evils due to thie early exhaust at onoe become apparent, and are plainly llluetratee; in the annexed sketch, Fig. 1, iu which I have placed the cylinders side by side, and attached their piston roda to the ваше cross-head, so that they make thair strokes simultaneously.

I do not know whether this is the arrangement of "Relivot's" engine or not, but that is of no importance, as it does not affect this discussion in the least, and moreover it Is a oommon way of constructing compound engines.

The cylinders are drawn to a scale of half-inch to the foot, but the valves and ports, for sake of greater distinctness, are exaggerated to tbe inch scale. The

Elstons in both cylinders are at the point when emanation takes place, viz., 78 per cent, of the stroke, and they are moving iu the direction of the arrows,, and both valves are at the middle of their excursion,' just closlug the exhaust en one side, and opeulng 1ц. on the other side of their respective pistons. That steam from end A of the high pressure cylinder is passing away inte the conaestlng pipe and valve, ohest of the low pressure cylinder, thus undergoing considerable expansion and reduction of pressure at tho vary time when It should be securely bottled up ia fbo cyliuder, and driving the piston at least 19 per cent, further on its way to the end of its stroke. At the same instant that the steam it thus leekk-slypermitted to escape from the positive side of thé piston A, compression be gins on the negative aide В for it will be seen from the position of the valve, that exhaustion from that side has just been Inter-! cepted; the piston, therefore, during the remainder of its stroke encounters a great resistance in gradually compressing the unexhausted vapour back Into the, port and small space allowed for piston clearance. To be sure, a part (and only a part) of this power is restored to the piston on its return stroke by the re-expansion of this compressed vapour. But why cómprese it at all? The evil resulting from doiug so can, be avoided by using a properly-constructed valve motion.

At the same time that the above bad work is going on In the high pressure cyliuder, similar bod manager ment and prodigal waste are golug on in the low pressure one, for the steam, which up to this instant baa, been driving the piston on its wny, is suffered to escape to the condenser from end C, Instead of brius; kept on. the piston at least 19 per cent, longer, and compris-ion. also takes place on the other side D, as in tho high pressure cylinder, for the commuulcatlou of this ond of the cylinder with the condenser is prematurely cut off, and the.vacuum applied to the wrong side of the pisto u.

But I have yet to point out the greatest evil resulting from the bad proportions of these valves, which la

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о be found In the restriction of communication between the two cylinders.

I have said at the beginning of this letter that one •f the essential conditions of a good compound engine is " that the communication but ween the two cylinders should be free and unrestricted from the beginning to the end of the Btroke," but how do we find it in this case?

The valve of the low ргеввиге cylinder cut? off at 30 per cent, of the stroke, that is to say, the communication between the two cylinders is improperly intercepted at that point, see Fig 2, which shows the position of the several parts at that*Instant. The low pressure volve F has just closed, and no more steam can pose from the exhausting- and В uf the high ргеаBure cylinder to what should be the receiving end С of the low pressure one The high pressure piston has to spend its energies in crammiug the unexhausted steam into the passage of communication and valve chest of the low pressure cylluder. per cent, of steam which has gained admission to the low pressure cylinder makes a faint effort to drive the piston by its expansion. It is buta faint, effort compared with what It should be, for it U unsupported by the great body of eteam which should have free ingress to its assistance, but from which it Is cut off—Isolated—by the premature closing of the valve.

If that engine were mine I should make the following alterations In it :—

I would make the ports in both valve clients exactly alike in their transverse dimensions, that is to sa/, I would cut ¿In. off each of the admission edges of the ports in the high pressure one, and ¿in. off each of the ■exhaust edges of the ports in the low pressure one. I

7 would then cut 2— in. off each of the admission edges

16 of the low pressure valve, which would leave it \'m. of lap, and I would make itsexhausted^es 7fcin. apart, or the same as the exhaust edges of tho ports, and I would then reduce the travel of the valves from 7iin., ав at present, to Sin., by putting on a new eccentric, which should drivo both valves, and this eccentric should be set to give the valves ¿in. of lead. I would do away with tho present high pressure valve, and introduce a compound valve, as shown at Fig. 3, which consists of three pieces, viz., slide valves G and II, and an interpose 1 plate I. The valve II should be worked direct from tho eccentric by rod J. This valve Is made with an exhaust cavity K, whose edges are equidistant with those of the ports, and two admission passages L L' straight through its solid ends; the admission edges of this valve have | of lap, the same as the valve of the low pressure cylinder, and of course the same lead. The interposed plate I has two ports cut thronst h it exactly opposite tho steam ports in the cylinder face, and of the name dimensions; this plate lies loosely upon the back of valve H, and valve G lies loosely upon the back of I, and all three are pressed up together by the steam. 1 is prevented moving from its proper position by studs or stoppers fixed to the aides of the valve chest. Valve G le a plain solid plato, having tin. of lap at each eud beyond the ports in plate I; this valve is worked by a " link" M, one end of which derives Its motion from a rod X, pinned to

the main eccentric rod at O, and the other end is worked by n rod P, from a small eccentric Q, fixed on the shaft beside the main eccentric. Q has 2Jin. of "throw," and it is keyed on tho shaft with its centre line, making an angle of 158° with the crank (I am now speaking of a horizontal engine). The effects of this construction are—that when link M is lowered so As to bring eccentric rod N into line with valve rod K, the movements of valves H and G will be very nearly simultaneous, aud steam will not be cut off till the piston has performed 95 percent, of its stroke, and exhaustion wilt take place at УГ per cent. But wben the link Is moved from this position, simultaneous

action of the valves ceases, and the movements of valve <} become in anticipation of those of valve H, and this Anticipation increases gradually till eccentric rod P Is placed in line witü valve rod K, when steam will be cut off at 10 per cent, of the stroke, or 2jin., and anv Amount of admission between these extremes Is to be found between these two extreme positions of the


It will be seen, therefore, that this simple apparatus possesses the inestimable advantages of a variable cut off between very wide limits, united toan invariable exhaust taking place at a late period of the stroke.

The effectof these alterations en " Kelivot's " engine would bo that the communication between the two Cylinders would be kept open during 97 per cent, of the stroke, instead of being prematurely cut off ns it Is now at :î6 per cent., and the steam by its uninterrupted expansion between the two pistons would continue to do good duty upon the large piston for fll per cent, longer, at the earn-* time relieving the small piston from the enormous back pressure which at present paralyses its action and reduces its useful working effect. Also the communication between the low pressure cylinder and the condenser would be kept open during 97 per cent, of the stroke, instead of 7S per cent, only as at present, and its piston would, therefore, continua to receive the full bone fit of the vacuum for 19 per cent, longer.

If " Relivot " should wish to hear from me further, 1 shall be always at hie service, with your permission, through the medium of the English Mechanic, and I shall not again keep him waiting four months ior an answer. Jas. Uaskervillk, 5, Westland-street, Limerick.


Sir,—In reading over "F.R.A.S.'e" letter, it gave me great satisfaction to hear him speak so well of Mr. Wray's object glasses, as i have a 4in which I got from him. On Jan. 24th. 1809, I had the pleasure of observing three occultations of the «tars 115, 11!', and 120, in Tauri. ТЬлче stars were quite bright and clear until they disappeared behind the moon. In July 1 had some good views oí .Saturn, although he wae low at the time. Venus has shown very sharp, so much so that some of my friends thought it was the moon that they were looking at. On Jan. 25th, 18Г0, and on March ftth, I had the pleasure of seeing six stars In the tnipezium of Orion, and two small stars a little below in the nebulie

Since I have rend'' F.R.A.S.'e letter," I muet acknowledge that I am proud to be the owner of such an instrument. I think that 1 was the tlrst who rnade mention of Mr. Wray's ot ject glasses in the English Mechanic, some time ago; and asan act of gratitude to Mr. Wray I cannot refrain from writing, as his name is now before the readers of the English Mechanic. If not trespassing too much, I wish to say a few words of encouragement to my fellow working men. When I first made a start, 2*. was all that I could spare lor an object glass, and no English Mechanic to help me on—no "Chats on the Stars." 1 had not the privilege that the working men have now of gettlngevery question answered through the columns of the English Mechanic; but perseverance has done more for me than money could ever have done; end now that I have overcome the difficulties that I have had to contend with, and am in possessionof a good telescope, I do not wish to put it under a bushel, but I am ever ready to bring it out to any one that can make it convenient to call on me. Working men, think of this; and let your motto be Aïf de*perandumt D. Alston, Clyde-street, Merefleld, Rochdale.

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Sir,—I take the liberty of sending yon for publication a plan for this purpose, on a scale of l-12th, capable of doing real work, and suitable for driving two pair of usual 4!t. stones—or, say, three, as one may be frequently up for repair. The sails are enclosed in a wlndcap, because the open horizontal windmill cannot produce profitable work, on account of the wind only catching one sail at a time, and that for an instant only, besides the Insuperable difficulty of trotting the sail frames back against the wind; whereas iu the common vertical windmill all the sails act at the »ame time, aud moving edge on, atmospherio friction Is very slight.

In the horizontal enclosed form It will be seen that five sails ore always acted upon by wind concentrated by the funuel-like openings. The latter are 24 x 15,

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wind there will always be a partial vacuum on the le* side, which acts as an exhaust, so that nearly the whole of the concentrated blaut is available for work..

The mill-house must be, of course, a substantial brick building, either square or round, nccordinr to fancy. Tlie wlndcap above may be of thin board, out the joisliug must be strong, iu order to stand the strain of tho upper gudgeon.

The regulation I* a v«ry simple matter, because the twelve sails are on rollers, which can tie reefed er let out by hand gear round the main shaft, or better, perhaps, by means of a self-actiug governor. The

Kressure of a good working wind of 20 or 30 miles an our is found to be from 3 to 41bs per squaro foot, ox at least at double that upon the sails.

Henry W. Revelev, Reading.


Sir,—having for many years taken great Interest In the study oí phrenology, 1 beg you will allow me to say a few words In reply to eonie of your correspondents, who have, through great ignorance, endeavoured, to throw discredit aud ridicule on the science. It is a great mistake to say that phrenologist« assert that big heads have always the most power. All true phreuologists maintain simply that size le a measure ot power, with this reserve—viz., all other things being equatt and they will Invariably maintain that temperament or the quality of the bruin is most essential to the effective working of tho organs; therefore an avcraged-sined head with well-proportioned organs and an active mental temperament will produce far greater and better results than а large head with ill-balanced organs and a slow lymphatic temperament.

I make this statement from personal knowledge and many years' experience. I know a man who has a head 23iu. in circumference—which, by the way, is а very large fize—and although he has had many advantages, hoe produced Utile either good or great; and on the other hand, I know a man with a head only 21)10., but the piepondcrance of brain being in the upper region,and with a most remarkably active mental temperament, who has taken a most prominent part in thr political and literary world.

I may observe, for the information of those little acquainted with the subject, that temperament It, in regard to the human brain, much the same as blooa or breeding Is to the horse. Self-mude men bave, I believe, Invariably big heads or remarkably active temperament. ¡э- H. Л.

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th segment dampers, such as »re put on oreo door» to vent out, or regulate the heat of the etove.

Fig 4 representa a email portable »corn-.baped heatlag stove, having but one burner. This stove U

.two niece« <• and e. Tho part ■ hna a number oí smaUiin holes level with the bottom of which is

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F Г„- 3. Three holes oi sufficient diameter are cast m
these plates, two in the plate с aud one In the pi»'««,
wíih lid" ft Aft Figs. 2, hinged Into the top bed
Of the stove The two doors ft i are laid back on the
bearance. b, when the cover plate e e t is itted off by
the ring i- to set in the large bo.ler for washing
r-li.vintr m»v be going on In the oven m and on the
1 back burnc/o K-1 • »nncepan, may be set at the
„me time the washing is going on. The centre of
,BTMe hole, in the portable top plates, for «tewing,
frying, *C must be direct over the gas burners. 00
«re tfie two front gas cocks, and . s the two front air
cock, into the burner g. The air-vent» « w arc stnal
hô^es about j|n. diameter, one to each burner the
hole« V, which may be each lia. diameter, hole, fitted

с string, tuned to the same pitch, may diner
Irom another in thickness and length (and consequently
ts sounds difteriu timbre., so the sound "1 one reed
which i» thicker or wider in pioportion to its length,
mav differ from another reed in ils sound, not to men-
HÓ, that the character of the Bound», which are caused
by the same reed, may differ greatly according to the
treatment to which it is subjected, for in this respect
, rn о ..uni reed» resemble human beings, and utter
very different sound., according as they are ice« or i«

ТЬе moet obvious menne of varying what I may
term Ihe original sounds of harmonium reeds is to
vary the proportions their h-ngthsbear to their width».
It ii hardly to be expected that the sound of a reed
which is only twice as long as it is w.d ■ w il very
closely resemble tbr-.t uttered by another whose length
is from eight to twelve times its width any m'lrethan
we can obtain souuds of the »ame timbro Irom the
long narrow pipes of the dulchma rank identical with
tho-e uttered by open diapason pipe, of more than
double their diameter. The character of the sound,
we obtain from each will be in proportion to their .eo-
tional areas, like producing like, and we might ш
reasonably expect to gather grape, from thorn«,.or
fi-s from thistle., a. to obtain tho full broad tone of
the wme scale open diapason P'P* f«TM/';^»TM*
dulciana, or the beautiful quality of tho Utter from
the former. No doubt the shilful volcor can do much
,„Ы» mouift», but neither manipulation nor
cnem .try c»n yet wash the blackamoor quite white,

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without also washing out hla Individual life; both ho and the pine whose tnonth in opened t*to much are only lit to go "down among Liu- dcadnuju,"—}uu can't well make them -inn up.

Besides that variation of quality of sourde, induced by varying the proportions of length to brealth, different méthode of affixing the reeds to their frames, and different méthode of forming the rec s, considerably influence their timbre, Good workmanship can do much, hut, According to tin- Ute Mr. Myers, nethtng can compensate lor the Injurious consequences of the reed and its frnme beiag formed out of two pieces. По used to my, which is true, that absolute contact between them was beyond human art to effect, and that an uncertain amount of chattering luvariably ensued when tac reed was simplv luid to its frame by a saddle or cross bar, screwed down on it Also, that the beet rivettiug did not prevent cliatteriug If the reed was made of thin mateiial instead of being forged out of metal fi ui l-UHh tu 1 14th Inch thick, and тегу slightly shouldered ou the aide nearest Us frame. I have found this a wry effective method of preventing chattering, but Mr. Myers had a ureat faith iu his patent method of construction., iu which the reed is cnt out of the same subs tunee of its own frame, and only rem .int. attached to it at the fixed end. That hie eoiophons, with hardened aad spring tempered steel reeds thus formed, produced tones of far greater purity than any of their contemporary seraphlnee, I can avmch, but that this wae entirely due to their reeds being made of the ваше plec of sieel which aleo formed their frames, I ean't help doubting. I suspect good workmanship had something to do with it; but whatever the cacee or causea might have been, the effect was excellent. The eolophon differed from the thin reedy t-craphine nearly as much as a fine organ reed, employed in uuison with the open diapason, differs from ye ancyente oboe, or vox humana stop, voiced à la Punch, whose dulcet sounde ye ancyent voieer of pipes evidently considered the ne phis ultra of reed tone.

Nat loop since I heard an harmonium, by Ramsden, whose strident tones might have satisfied our MeChanic's princesa " Alexandra1' herself. This instrument, which drowned «very other iu the room, had but one and a half row of reeds, whose tonee re ru ark ably resembled some of those German concertinas which so greatly surpass their Knglisb namesskes in power and—It I may be allowed to com an adjective—also in horninesa. Pos>ibly great pressure of wind had something to do with the matter, for 1 find the steel reeds of my Day and 51 u иск'a eolophon assume thi* quality when forced. I suspect the pbysharmonica with a ■well (not, of course, the organist himself, although some of them are both great swells and heavy swells to boot), with its free reeds blown by 10 or 12iu. wind, has something of this strident character, and sounds very unlike the tender reeds of an Kvans'sor Hermann Smith's chamber harmonium, which are delicately blown, and perhaps also—as all good tender children should be—well wrapped up to keep them from squalling.

This wrapping up is a very effective method of subduing unpleasant sounde. Every paterfamilias knows —to bis horror—that his lnfaat'e eries are strident, Indeed, if heard in their native nalcednese, but become quite bearable when Its dear little head le under the bedclothes. It won't do to let the infant breathe too ffttly, or he becomes "the son of the sleepless "; and eren ff this mode of quieting sometimes be fatal to it, we have at least the consolation of knowing that its departure to the world of spirits diminishes that pressure of population on the means of subsistence which political economists so earnestly deprecate. No doubt the reader will say there ie nothing new in this, ft having lone been familiar to " ye family man," but if not new itself, we have not long since become acquainted with a new application of this тегу old fact, which new application well deserves letters patent not only to be granted gratuitously, but also that its inventors should be fully pa'ui for their ingenuity. I need hardly say that I allude to the late Shoredltcb iufirmary practice, in which a noiay pauper —what right can paupers have to make a noise ?—was duly and effectually quieted by having a cloth forcibly held over hie wicked mouth, while the mm prepared a sufficient dose of morphia to quiet him effectually until the day of resurrection, on Count Fosco's principle of assisting tired Nature's tendency- to permanent repose. Verily we do improve in the treatment of insane paupers, for not only do we liberally proTide wet cloths for their crying necessities, but also expensive medictues, dispensed with a liberal hund by muses, to assist their .progress out of all mundane miseries.

To return to our dead sheep (muttons, as the French вау). I very strongly suspect the reeds whose draw ■topeare duly labelled clarionet, hautbois, and most of the other varieties of timbre we hear from the harmonium (or rather oug/it to hear). Too often the real tones are very far indeed /rom that instrument, are much more due to the different areas of the spaces between, the reeds aud the pallets (within which spaces the air, under the influence of the reeds, becomes resonant), and the quantity of what I take the liberty oí terming bedclothes (interposed between tbc reede and the hearer), than tn any other cauces—of course I mean the extent to which the rceda are covered up. t am told the popular voix celeste—which can only be termed celestial ou thehypotbesistbat tbeangels practice singing the music of the spheres (whatever that music may be), with remarkably thin reedy voices—is produced by another dodge, but instead of enlarging and perhaps yet further exposing mv own ignorance on ibis not very celestial subject. 1 would recommend the reader to peruse Mr Hermann Smith's paj er on it, for an expert can. jf he wilt, almost invariably give us more Information than any mere amateur, however clever be may m&ke the very common mistake of thinking himself.

Bat few modern harmoniums which I have seen are constructed with the means of blowing their reede with wind of different pressures, without using the expression stop. Myers and Storer, 183У, and Hewett, 1838, patented methods of doing this, with only one bellows, and, I believe, others have sine© invented other ways of effecting' it. The thing seems to me so very desirable, especially for instruments with

two or more rank* of keys, that perhaps "Eleve" (who te no eleve at all in the matter of harmoniums, excepting in the sense that the wiseat of us have something to learn) or Mr. II. Smith would kindly express his opinions on this method of vavylng both the power and quality of the sounds of tuu instrument, tor, if desirable, it is extremely cheap, enly requiring one or two extra weights to be put on the bellows at the performer's pleasure, aud this may be readily done by disconnecting tbelr »apports; or if the ase of weight be prohibí ted, by allowing additional spriuge to come into action.

That any free teed instalment will ever render that wide variety of timbre obtaimib'e front organ pipes Is hardly to be hoped for. IГ anytbiug at all like it could be done, the instrument would well deserve the name of wind organ, but I am tar from believing that our ae-called wind iu trument stop« whose names are engraved on its draw, exh*mativuly represent the capabilities of reeds for producing aginada of different timbrée. I suspect It is rather because no one has yet shown what can be doue, and how to do it, such an Instrument has not been made, and that no commercial demand exists for harmoniums with greater variety of tone. "Alexandra" only a*ke forwhatcvery organist who performs on a tolerably large luetru* meut with a swell, already possesses; and if she can induce some expert, cheaply to supply her requirements with free reeds, which at present are only to bo supplied bv pipea at great origioal expense, not to mention the coat and trouble of the oft-repeated tunings which a large pipe wrgau with many reeds requires, she will have the satisfaction of having caused an addition to the number of the good things of this world to be brought into existence, and for one I heartily thank her for asking for it; indeed I consider her request an exsmple of that refined musical taste which invariably distinguishes her sex.

Tul Harmonious Blacksmith.


{Continued from page 11.)

Sir,—The telescope waa next directed to the two nebulas before-mentioned. We shall now ouly give our impressions of the latter (И. 13Herculls); and what a won der lui sight we beheld when it was brought into the field of view! With a low power a small, bright cloud, all sparkling with diamond points of light, but with a higher power how magnificent the change I Stars piled upon stars, not one trace of the cloud remaining, but star-dust, even to the very centre, where they seem so infinitely close, that one wonders they each can still be seen separate from the other.

Here le a test for the perfection of figure of a speculum, than which none could be more severo. To completely resolve this superb cluster, oven to the very centre, with such an aperture, is work any man might feel proud of. On the second evening 1 examined this object most carefully, and for some time, and could most distinctly trace the two dark rifts first seen in the Earl of Kosse's great tele*cope. They were best viewed with the highest power I used (3u0).

But enough hue been said to show the extreme excellence of the speculum 1 was employing, and the sufficient adaptability of the mounting described in a former letter. Let every amateur take courage, and if he only possess a little ingenuity and patience, be may do even better than 1 have done. The above is but a short account of one or two nights1 work—very favourable nights, I will admit, and such as do net come very often, but when they do amply repay the earnest student any small addltloual outlay he may have made iu p.rocuriug a really good speculum or object glaaa. Whatever be does, let the beginner beware of cheap object glaase* or specula; iliuy always break down wheu such "testing times" come, aud prove ultimately a loes rather than a gain.

I would rather possess a xeally good ¿Jin., or even 2ln. object glass, than a cheap 3fn.; and this reasoning holde good, a fortiori^ with regard to specula of respectively larger aperture, as with them the evil is increased in a threefold degree.

There Is one fact, however, which I have learned from constant observation; it is that when the air is unsteady there is no use in trying to use an aperture of8£tn. ; perhaps this may seem too strong nn assertion to some, but certainly the position in which my telescope Is placed—a position very similar to that described by Mr. Purkiss (near a largo city—has taught me (its truth. Many nights come when I can use emaller apertures, and see planetary and lunar details sufficiently well to learn something of phenomena taking place In those bodies, which a larger aperture would not even suggest.

Nights often come when it is vain to attempt to divide even such a star as a Herculis with the whole aperture, or see the transita of Jupiter's satellites, yet a leeser aperture of S or «in. will accomplish both tolerably well; unfortunately too, those are the kind of nights which predominate in our climate. This I say with great deference to Mr. C. Key's opinion, which is deserving of the highest respect, but it has forced itself again and again on my notice, and were 1 asked to advise one of your correspondents who wae anxious to possess a telescope which he could use on most bright nights, and was not willing to keep a large one in reserve to use on good occasions only, I should strongly recommend a refractor of from 3 to 4Jln. There le a calmness of definition In a refractor, arising from the air between the object glass and eye-piece being ¿nut In, and nearly free from disturbance, which has greatly tended to Induce a preference for that instrument over the reflector. This freedom from air currents is frequently mistaken for a greater sharpness of definition, whereas it maybe stated with all confidence that a reflector, if it be a really good one, is, on favourable occasions, equal to the very best refractor of the some aperture, only those favourable occasions do not como quite so often with the former as with the latter.

The position of the eyc-pirce In the Newtonian telescope, as it Is generally placed, has something* todo with these tube currents, for the heat from the observer is almost necessarily brought In contact with the air In front of the tube, and with the surface of the tube itself; it would be very difficult to estimate accu

rately the exact ratio of disturbance originating frl tbls sonrce, as it varies во much under different ci cum<*tauces, nor can we hope to eliminate it entirely until the good old form of the Cassegrainian | or Gregorian telescopes come again inu> general use. open tube could then be used with o,reat advauta,_ which seem« rather to increase the disturbance from this cause in the Newtouian télescopa

I hero is, however, no compa>Lson whatever in the relative greatnees of this sourcu of iudisiiocineaa, between the Newtonian and front view construct/ Of all the forms ever derieed, the hut-meo_ telescope is во completely at the mercy of troublesome cyclones that, leaving ont of con»ideaav tton the spherical aberration, sharpness of di.-(initios is almost, if not quite, impossible, in such a telescope — certainly, if the correspondent, who tent you a -kc'th of his telescope lately Is the happy possessor of one of those large heads, so highly кр-kuu of in a recent number, very little light, and that of a very tumultuous character, can ever reach kls l&ifl. mirror.

While speaking of this form of telescope, wae sorry to read the abrupt manner In which your talented correspondent "1ML A.S," took up a remark ef "Arcturus'" with regard to spherical mirrors. Your worthy correspondent has always been so eonrteous in his replie«, and so ready to assist us at ail times, that we should expect, for one who has taken so much trouble to enlighten eur minds ou tbc mechanical minuties of speculum grinding aud polishing, and to teach us lessone wbich aro rarely, if ever, learned from professional astronomers or optician*, better treatment at his bands. 1 cati hardly see how "F.K.A.S." would, in any degree, trench upon to* just estimate he has formed of the absolute necessity for the parabolic curve, if true definition is w В attained, were he to confess with Sir Jobu H erseht*, "that for this construction it Is nee deas to insist am the parabolic curve." My own experience is. that (be spherical aberration of a mirror, parabolised tn «Its» usual way, is quite equal to. If not greater, than that» of a good spherical one, wbeu tilted at an angle oftf» and 4 degrees, and that experience was learned from % most perfect 4£in. speculum by With. For рими i i Iff. the sUver films of these glass mirrors, if the observes does not wish to remove them from tho telescope each time ho has finished using it, I have found no plant «• efficacious aa to turn the tube ii-elf into an air-tight cell; this Is easily done by having it made perfectly stanch In alt its seams, aud having a lid for the топая, which fits most closely and ueatly; to this lid i» attached, by a string, a long roll of some light ГиЬНе> which baa been steeped Id chloride of calcium, аи thoroughly dried in an oven or at the fire, before Б serting it into the telescope. It will quickly absorb all the moisture in the tube, and as no more can creep In so long a« it is closed, will keep the -ilver surface perfectly dry and bright. I have often thought, bowever, that some of your clever chemical correspofrdents would be conferring an incalculable benefit oa> the possessors of three telescopes if they could discover some method for precipitating pure aluminium on glass; It is of an exquisite white colour, of very close grain, and receives a higher polish than i ven silver itself, aud might tend to render thrui quito equal to refract« rs in light -gras pi ng pi» we/, aa they are at present in defining power.

I see in a letter from one of уодг excellent practical correspondents, that he paid £'2 for a "focussing metion " to the eye-piece of his telescope; now for about one-sixth of that Mim he could bave procured a { plate lens, secondhand, with rack motion. Having taken out the lenses, he could eaaily bave fitted the spring-tube of his ove-piece to thia, and thus have obtained a most excellent and "imple focussing motion. This plau la not mine, as 1 feel sure many of your correspondents have seen telescope« so mounted.

The most pleasant "finder " possible, can be formed by placing a rectangular prism before ihc eye-piece of the finder, aud so turning the raysout parallel to those from the large mirror after thoy pass the "plane." By this plan you have only to turn your eye from one eye-piece to the other, while still reclining in your chair, and need not etrain either your neck or your eyes to find what you may wish to observe.

I cannot close this letter without two words of thanks, first to yourself for the successful manner hi which you have conducted your most excellent journal of late, and the abundant store of really useful infor

mation you have managed to obtaiu for its Р*§Щ from those whose "fame Is in all lands,'* an I freronilv, to your generous and patieut correspondent, Mb

Purkiss, who has made a real dNovery iu the seien©* of speculum testing, and one of the very greatest value to all those interested In the r-ubjecL I haw tested it myself, aud believed It possible, with due) attention to particulars, to attain, by the means he suggests, a mirror of any size, perfectly free from spherical aberration.

P.S.—I am veey glad to see Mr. Wray's nam* brought before your readers in a recent number by tho Rev. Mr. Webb. I have two object-glasses by blm, one a 2 8-10ths Inch, already mentioned, thai most perfect of its kind I have ever seen, the other ft 4in., recently obtained, which quite surprised me by Its light-grasping power. Oa two occasions 1 hire distinctly seen the fltb star in the "Trapezium" of Orion, a test I used to consider far beyond such an aperture. I do not mention these matters by way of advertisement, but to assist these In search of re" good object glasses, and to whom the price is a J Important consideration. If the amateur can savei in the purchase of an object-glaes, and obtain a rear good one, he has the money in hand to purchag battery of eye-pieces, no mean considération afters;

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