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eed by giving lome farther particular* relative to | A drum ia fixed upon the axis of the puller at the edge of

'- the table; over tins drum is stretched aaheet of paper, and,
during the motion of the puller, a pencil whose point ia
pressed against the surface of paper ia cauaed to more with
uniform Telocity parallel to the axia of the drum.

Aj the pencil haa thus two eimultancoua motion*,*

m directiona at right angles to each other, a curre

must he the result, and if thia curre be a parabola, it

a eaaily shown that the acceleration of a point on the

drum, and therefore of the eliding body (since the cord

moves with the aame Telocity aa a point on the drum), must

be constant, and cooaequentlv that friction ia inde pendent of

the Telocity; for if the motion of the eliding body be uu iformly

accelerated it must be acted upon by a

constant I force, and therefore the friction

cannot rary with the Telocity.

In Fig. 11, let A B E be a portion of the parabolic arc deaenbed (for in the experimenta a parabolic arc waa clearly obtained), whoae axia. A D, ia at right anglee to the axia of the pulley, let B be the position of the pencil's point after the interval of lime (from the commencement of the motion, draw B C perpendicular to A C, a tangent at

A. then, by the well known property of the parabola, if p

be the latus rectum

tie ertnieTaJ caodea of producing ice. 1 he materials suitable tor abstracting heal from water in sufficient quantities to be n^e^ikr for reducing temperature were, liquid ammonia, niraSai and ether. The substance hitherto uaed for proAr« cold had been almost entirely ether vapour, with the eart»Qott of the expansion of air, To produce a lb. of ice, we rweired to take out of water at a temperature of 60° a qusntavr of beat repreaented by 148 heat units, and to cool the water from the temperature of 60° to that of 38°. Speaking cd this in larger quantities, the lecturer aaid that 3i481h. of -tvbex reqairrd to be vaporised in order to convert one ton of water al 60° into ice at 32°. Although there waa a great .difference between the quantity of thoae materials which were rtqaitite to produce thia change, experience went to anew tiat the solution of ammonia waa the moat efficient ■uvter.il to be used for absorbing heat from water to convert i: lato ice. Still, ether had the advantage of being since pfe'Mc to vaporisation at low temperatures. Neither water nor a!coiol would Taporise at a low temperature with anything hie that degree of rapidity which would suffice for artificial cooling. The doctor then deacribed the way in which ether waa aaid. It' might appear remarkable that fuel ahould be used for producing cold, but the reason waa, that the rate ■of evaporiaation that corresponded to the tension of ether ■would be too large to (.reduce any appreciable cooling, or •ufficient cooling of water to produce ice. Therefore, the enporisatitm of ether had to be facilitated by mechanical ■scans Willi the aid of a diagram the lecturer explained the whole of thia process, and then spoke of the ether apparatus A machine capable of producing five tone of ice in the space of 24 hours consumed a quanlitv of fuel which waa in the proportion of lib. of fuel to 31b. of ice produced. The tension of vapour varied according to temperature, and that had an important influence on the efficacy and work of this material in the production of ice. That, in brief, explained -the way that fuel bad to be consumed in making ice by means -of ether- Ammonia, at an ordinary temperature, waa a gaseous substance, requiring a pressure of about 1501b. on the square inch to reduce and maintain it in a liquid form. If we used ammonia to produce ice, it must be al the expenditure of a roniivierable amount of power and of fuel. When we Rmemtit'ed that in the ateam engine there waa not,

feneraJIv speaking, more than one-tenth of the beat produced y coal burnt which became affected by mechanical force, it must be evident that this mode of working ammonia must be very cortiv; and so it had been found to be. But. on the other bam!, there was found to be a faculty characteristic of ammonia which rendered it possible to do away with this application of mechanical power. That character was the faculty of ammonia for being absorbed into water, and this characteristic had been taken advantage of in working tbe .ammonia machine, aa he ahowed by reference to two die grama of ammonia machines used in the production of cold. In both a strong solution of ammonia was separated, and used over again; bnt the special ,*—■•-- "■■

liquid ammonia waa obti
in the other. The expansion
thia purpose; but this was so

TefnEeration, aa almost to place its use out of theqneation.
Axtiflcittl refrigerators were now in use, and he had only
lately heen informed that in the breweries of London there
-were cooling machines working which were capable of cooling
an much as nine hundred barrels an hour to the extent of 10
degrees, which waa equivalent to the cooling of 324.0CO gal-
Iona an hour, and that qoantitj cooled 10 degrees was equi-
rafext to the melting of 10 tons of ice an hour. At this
point Z>r. Paul entered upon that part of bis subject which
should have commeocei his lecture. He began by showing
tbe uses of fuel for the production of high temperatures, snd
observed that the temperature produced by the combustion
of fuel depended much more upon the way in which the heat
-.generated was disposed of in the production of combustion
than it did in the actual amount of beat This wss espe-
■eialiy important in the use of fuel in such operations as the
jnsnufaclure of iron. Tbe specific heat of almost all auh-
stauces increased as the temperature increased. In the case
-of iron it was more than double st that high temperature
'required to be maintained in the puddling furnace. The con-
Suuvpikju of fuel in puddling iron was at the rate of a ton of
fuel for every ton of iron produced, or something like 18
times as much as the quantity of fuel repreaented in the
actual work done. By thia he meant the heating of the pig
iron up to the temperature necessary for producing the
change required in puddling and for melting iron, fuel,
under existing eh-conistanees, was indispensable. Speaking
■of gas, the lecturer said that the specific heat requiaite to
waise the temperature of lSlb. of gas 1 degree was sbout
three heat onits. In the furnace three-eighths of the heat
was used, and five-sighths passed off in waste; and this pro-
portion applied to glass-making and to all operations necrs-
nary to be conducted st a high temperature. Tbe higher the
temperature produced, tie greater the amount of free heat
there was available (or the work to be done, and the greater
was the economy of fuel. At high temperature there was a
rapid waste of heat by radiation and conduction, and by escape
in various ways. The temperature regulated the amount of
-work done. Dr. Paul next alluded to the use of fuel for
producing high temperature for the production of iron in
what was called the smelting furnace, the conditions of which
prevailed in the hot air blaat furnace. By means of a dia-
gram he showed the difference between ordinary combustion
and that taking place in this furnsce. In the smelting fur-
nace, carbonic acid was converted into carbonic oxide, which
-was combuatible, and which furnished heat snd effected a
saving of fuel. In many cases the consumption of fuel ran
•up as much as three times the weight of the iron produced;
and this large expenditure of fuel was attributable to the
•sum oases which operated in the case of tbe puddling fur-
nace- There wss reslly only about a sixth part of the fuel
-consumed which was turned to account in doing the work of
reducing iron. The plan of burning fuel so ss to obviate
<hi* waste consisted in burning it sfter having inverted it
into a gaseous form, ss in the case of the production of gas.

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but — is constant, hence B C bears a constant ration to P, or

p
the spaces traversed by a point parallel to the axia A D of
the parabola vary aa the squares of the times of traversing
them ; bnt a point under the influence of an uniform accele-
ration traverses spaces proportional to the squares of the
times, therefore a point which describes a parabola must be
under the influence of an uniform acceleration.

Let/be the dynamical measure of thia acceleration in feet

per second, and we have C B = t / <*, »y the relation

/l|' 2 a»

between space and time, also C B = therefore/ =

J>

In experiments such as the above, where acceleration takea

place, it is clear that the descending weight exceeds the

friction by s certain quantity ; we must therelore find an ex

preasion tor u in Ibis case in terms of known quantities. Let Q

he the sliding body drawn along by a descending weight P,

then we hats the weight P + Q set in motion by the force P

moving force is P lessened by the friction

acceleration, as found above, we must

motion, P + Q: P — p (Juji/t,

%here g is the dynamical force of gravity expressed in feet per

second. _ „

tti-f) - Qf

Therefore/s = ———

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Example 8.—A mass of csst iron,
along a horizontal plane
cord which is parallel to the pla

drawn B

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weighing 1001b., is lane of cast-iron by means of a

,.. _„e plane, and to the end of which

a weight of SOIb. ia attached. Determine the coefficient of
friction, if the acceleration, found as above described, be ljft.
per second,

90 X (32 -ID- 100 X \\
Here u '

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100 x 32 x 3 9800 20

In any examples which may he given, g will always he
taken to be equal to 32.
We can also put the above expression for p under the form

T 9-/ f

Let Dow s be the -pace described by the sliding body "*' seconds, then by the well-known relation between the time and space described by a body under the influence of an uniform acceleration, we have

fP 2j

# => or/ = —

2 fl

Substituting this value for/in the above value for p we have

P g.» - 2i 2j p „ _ x

Q gP gfl

Example 9.—If in Example 8 the weight describes 6ft. in
S seconds,what must be the coefficient ot friction?
20 n x 32 -■.,■ >- 6 ?x 6
Here u =

include every circumstance of the case. As friction considered as a resistance constantly retarding tha motion of bodies has now been introduced, I shall here give the solution of an useful problem respecting the motion of a body along a series of inclined planes, and it may here be remarked that all the questions which have been given on the friction of sliding bodies apply generally to that of rolling bodies, for it is clear that the resistances to the motion of carriages, as on roads and railways, due to friction are made up of rolling snd axle friction, which together may be taken to be some definite proportion of the weight of tbe carriages. The resistance of friction on roads and railways shall however receive a separate account hereafter.

The following is the problem :—

k body hai ing descended one incline of gtv en length and inclination, is stopped on its descent or H#cent) on another also of given length and inclination. To find the friction which stops the body.

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In Fig. IS, let the body start from a, and, having descended a ft, let it come to rest st d, s point on the incline be. Let 0 snd 3' be the inclinations of a b and ft e to the horizon. Now if W be the weight of the body, W (sin. ,: - ,, cos. (3) ss haa been before explained, ia the force which urges the body down the plane, or, dividing by M. the maaa of the body, we have, aince W=M},} (sin 3 - p cos. 3) for the dynsmical acceleration of the downward motion, therefore, if t> be the velocity of the body on arriving at ft, and if be the length of a 4, we have r' = 2 g (sin. 3—p cos. 3> j, by the relation between velocity and space described. Now if we suppose the body to pass from a but be without lossof velolocity, we hare ita velocity on starting from 4 equal to v, and by. the nature of tbe question, this velocity is destroyed by friction when the body arrives at if, therefoie the friction, or p W cos. 3', is greater than W sin. f}', or the component of the weight along the plane 4 e. The dynamical retardation of the motion along 4 e is thereforeg (p cos. £}' — sin. .3'). and the relation between the final velocity (in this case o) and the space described when the body has an initial velocity, we must have

• = »> - Zg (p ces. p'-BB. 30 r"

or,

t> = 3g (p cos. 3' - sin. 3') el.

Kquating this value of r to that found above, and dividing across by if, we have

< sin. 0- i p cos. (3 = r" p cos. ($' - if sin. f}' Draw now the horizontal lines ft c and if h, and put a c *' j k = c1, c ft = i, and A d = x1; therefore sin. f3 - * i p cos. jS = ft *. ** = i', therefore « - p

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Bt C. Drurxn, A3., L.C.E.
(ConJimieof/rompocieeOl, Vol. X.)

Wbx w speaking of tbe tribometer, or apparatus fb* investi-
gating tbe laws of friction, it waa not shown how tbe law
■which declares friction to be independent of the velocity was
■vcDiied , indeed the experiments, ss then described, serve
only to prove the lews of the friction of rest.
Ibis law can be proved in the following manner.

Prom this last formula it will be seen that we can determine the coefficient of friction by observing the time the weight takea to fall any given space, without knowing the value of the acceleration.

In the above examples it should be observed that the following are not taken into account. The weight and rigidity of the cord, the inertia of the pulley and the friction of ita axis. They are therefore given to indicate merely the manner of experimenting, as the lormulm actually employed ahould

'Or, to speak more correctly, the pencil baa one motion, and the paper has another ; but tbe result is the same ss if the pencil hsd the combined motion of the two relatively to a fixed point on the paper.

t This statement is merely an instance of the law of motion that the velocities produced in the same body by different statical forces, acting for the same time, are proportional to those forces.

Example 10.—A body slides down an inclined plane the height ol which is 12ft. and length of base 2uft„ and elides along a horizontal plane at the botlom lor a distance ot oait. Find the co-efficient of the friction which atopped the motion, on the aupposilion that the body pssses from one plane to the other without loss of velocity.

Here « = 12, * = 20, and x> = 83
12 1

.•. p = —— -
20+62 6

The reader may verify the above result in the case of three
planes, howsoever inclined, snd indeed it is generally true
that when a body placed in the abave-statcd circumstances
stops in descending (or ascending) at a certain point d, the
value of the co-efficient of the friction that stopped it will be
found by dividing the total height from which the body de-
scended, by the total horizontal distance travelled over.

The above is also true for anv curved line, as the P'anea may be conceived to be so small that in continuation Uiey do not differ from one continued curved line. (To bt continued.)

THE ACTION OF MAGNETISM UPON LIGHT.

Fatbii Sxccni.the great Italian physicist, has recently laid before the French Academy of Sciences an account of Borne experiments he has been making conjointly wish Professor rruvenzali, upon the modiScationa produced by magnetism upon the light emitted by the rarefied gases. In theire xperinients thfy employed a large Bunsen's battery capable of giving sparks eight centimetres in length from the induction coil The gases were confined in Gciaslcr's tubes.

The results of their observations have been classed under

* is "whenever a tube of wide bore is introduced between or brought into tie neighbourhood of the poles of the.electromagnet, it is found that the diffused or stratified light winch fills the tube when a current is pssscd through becomes repelled by the poles, and is condensed on the opposite side of the tube, where it sppears as a bright •'»«*'M'"drt » luminous glow.' Indeed, the effect is as though lb<P«iW were really displaced, and accumul.ted in the nion. distaiit part of the tube Father Secchi considers »»'V'° "K; ince presents a. great similarity to the movement* of the streamers in the Aurora Uurealis

2nd. The condensed gas gives a well-defined and brilliant spectrum, and when the current is or sufficient intensity and the ess is one, sncli as chlorine or nitrogen, which gives H double spectrum, both spectra may be observed, the one in that part of the tube opposite to the magnetic poles beine, more intense than that in the main body of the tube. It Ik considered ss proved that the double spectrum is due to a difference of temperature, and not to impurity of the gas, as was supposed by M. Jiubruiufaut. A very striking result was present-d by the Indrogen tube. In this the light brought under magnetic influence became of a bright yellow colour, while the other part was of a magnificent roseate tint, the blending of the two colours strongly resembling in their shades the solar protuberances (red flames) of the eclipse of I860. The experimentalists think this effect

fartihtly due to the decomposition of the glass under the great eat of the current.

Father Secchi suggests that a simple solution of the phenomena observed may be summed up thus. Magnetism acta as though it contracted the bore of the tubes, and .by producing an elevated temperature causes the gas to give a spectra m similar to that which would be given in a tube of small capacity, and so it may be ascribed to diamagnetism. In conclution, he saya it is impossible to observe these pnenomena without comparing t cm with the magnificent phenomena of the solar chromosphere, and thinks that by similar experiments it may be possible to carry the analogy farther.

G. M. W. [We are indebted for the above communication to G. M. Whipple, Esq.. of the Kew Observatory. In regard to the first class mentioned, we showed precisely similar results in 1850 to Faraday. We produced the vacuum in an ordinary barometer tube, S6in. long, in the top of which a platinum wire was soldered, the mercury serving as the other conductor from a Rhurakorf. Faraday exhibited the experiment at a subsequent Friday evening's meeting at the Royal Institution, while lecturing sn the induction coll.—£d. & T. and K. /(.]—EUclric Telegraph and Railway Review.

A5TB0N0MICAL NOTES FOE APBIL.

By A Fellow or The Royal Astronomical Society.

The Bight Ascension of the Sun on the 1st of April will be Oh. 42m. 31s., nail his North Declination 1° 3V 30*. He will, therefore, be close to J" Piscium; and reference to our maps on p. 64, will show what constellations will in consequence be lost In his rays, and therefore invisible. The Equation of time to be added to the Instant indicated by a sundial, will on the 1st day of the month be 3m. 56s. It will diminish until the 15th, when there will be no Equation at all; the clock and dial being coincident After this the Sun will south before mean noon, or the Equation will become snbtraetlve, and on the 30th, 2m, 55s. must be taken from the instant of apparent noon to obtain the time, which a properly regulated clock ought to show. The Sun is now longer above the horizon, in these latitudes, than he is beneath it; the length of the day at the beginning of April being 12b. 53m., and on the last day of the month 14h. 42m.

The Moon enters her First quarter at 25 and a half minutes past 4 n.ni. on the 9th; is Full at 10a. 20m. on the night of the 15th, when, as she will be In Perigee, or at her nearest to the Earth, about half an hour afterwards, a high tide may bo looked for: She will enter her Last Quarter at 4h. 25m. on the afternoon of the 22nd :and be New at 6h. 37m. on that of the 30th. The Moon will be In conjunction with Jupiter at 49 minutes past 10 on the morning of the 4th : in conj unction with Uranus at 2h. 86m. In the early morning of the ninth : in conjunction with Sn turn at 3h. 40m. a.m. on the 20th : In conjunction with Venus at 9b. 53m. am. on the 26th: and finally with Mars at 4b. lorn, on the afternoon of the 9th. Practically the first and two last of these conjunctions will be invisible. During this month Five Fixed Stars, and the Planet Saturn will be occulted. At lb. 47m. In the early morning of the 9th, 56 Gcminorum will disappear at the Moon's dark limb j and re-»ppear at her bright limb at lh. 59m. At 2b, 15tn. a.m. on the 12th, 34 Leonia will disappear at the dark limb; to re-appear again at tho blight one at 4 minutes past 3. At 48 minutes after midnight on the 13th, 6 Virginia will be occulted by the Moon's dark limb, and emerge from her bright limb at 2 minutes past 1. Tho bright limb of the moon will oceirlt o Libra; nt 12h. 16m, on the night of tho 17th, and at 6 minutes past 1 it will re-appear from the dark one. The occupation of Saturn wi 11 take place during the early morning of the 20th. The planet will disappear at the bright limb at 2h. 65m., and re-appear at the dark one at 4b. 5m. Lastly, v'SagiI taril will beocculted by the Moon before she rises alter mldnighton the2iLh; but, after she has risen, will re-appear from her dark limb at 6 minutea pastl. The Moon's age on the 1st, at noon, is 0-4 days; She is 14 days old at noon on the 2nd, and so on. At 9 o'clock in the evening of tho 9th, Libratlon will bring a portion of the S.E. part of her disc into view; while at midnight on the 2i»t an additional portion of the surface in the S.W. quadrant will become visible from the same cause. These details, as has been before explained, are given to facilitate the observation of the various features of the Lunar surface.

Mercury Is prscticully invisible during the first twothirds of April; but becomes an evening Star, and may be looked for towards the end of it. During the las two or three days, he may be seen glittering close to the horizon, to the North of West after Sunset.

Venus is a morning Star during the entire month' Southing at 9b. 31m. a in. < n,the 1st, and nt 9b. Em. a.m. on the 30th. She is still a fine telescopic objeot, albeit the difference in the thickness of her crescent Is very marked when contrasted with the extraordinary tenuity which It exhibited is February. Mara is still too close to the Sun to be visible. Jupiter I* now visibly creeping towards the West. It is on the Meridian at Jh. 38m. on the let, and at lh. 10m. in tbc afternoon on the 30th. We shall soon be losing him as a conspicuous celestial object, for the season. He remains In Taurus during the entire month. Satnrn is a, morning Star until the 19th, after which be rises before midnight: be is, however, close to tho horizon, and wretchedly situated for observation. He is on the confines of Sagittarius. Uranus may still be observed before midnight; his position 1b Qeminl has not sensibly altered from that laid down In our little map on p. 470. Neptuue Is now invisible, in fact he ia actually in conjunction with the Sun about 8 o'clock in the morning of the 9th.

The Phenomena exhibited Jupiter's Satellites are very much fewer, the Planet's approach to the Sun interfering materially with their visibility. On the night of April 1st there will be n transit of the 1st Satellite. It will enter on to Jupiter's disc at Bh. 7m. On the 2nd the 1st Satellite will re-appear from Eclipse at 9b, 29m. 52s, but Jupiter will be very low down Tho shadow of the same Satellite will pass off the Planet's limb at oh. 4Jm. on the next evening : the Sun however, will only just have set at the time. Strong' twilight may also prevent the Emersion of the 3rd Satellite fr- m Eclipse at 7h. 19m. 59s. on the evening of the 4th from being observed. Ou tho 5th the Ingress of the Shadow of the 2nd Satililte will take place at 7h. 30m. The Satellite itself will pass off at the opposite limb at Sh. 20m-, and Its shadow may probably be seen to followit at 9h. 55ra., but Jupiter will then be on the Point of setting. On the 9th, Satellite I will be occulted at 8h 30m. On the 10th the same Satellite will leave Jupiter's disc at 7b. 52m., to be followed by its shadow »t8h. 36m. On the Ilth the 3rd Satellite will re-appear from occultation at 8h. 42m., only to be Eclipsed by Jupiter's Shadow at 9h. 29m. 26s. This is only some 18 minutes, though, before he will set. The 2nd Satellite will enter on to the Planet's limb at 8b. 4Sm. on the night of the 12th. On the Evening of the 17th there will be a transit of the 1st Satellite. It will pass on to Jupiter's disc at 7b.40m.,audita shadow will come after It at 8h. 18m. The first Satellite may also be seen In the twilight to re-appear from Eclipse at 7h. 4»m. Its. on the evening of the 18th. Fivemlnutes before Jupiter sets —viz., at 9h. lim. 33a. on the night of the 2Jst, the 2nd Satellite will re-appear from Eclipse. The 2nd Satellite will be occulted at 8h. 34m. on the 28th, or on the succeeding Evening, the 29th, the Ingreseof the shadow of the 3rd will take place at 7h.26m. : the egress ef the Satellite Itself occurring at 7h. 50m. We should perhaps add, that aa many of the phenomena, which we have included In the foregoing Hat will take place in strong twilight, or when the planet is almost close to the horizon, it will require a good telescope to observe them: even assuming, whnt Is problematical, that such an inatrument will render all of them visible.

There would seem to be some reason to expect Shoot lug Stars in April, during two intervnls j the first between the 4th and the 11th: the second between the 17th and 25th of the month. Watch should, therefore, be kept about these dates.

CONSUMPTION OF FROGS IN FBANCK.-The Echo du Luxembourg states that the exportation of frogB from that country;to France has increased considerably of late. Upwards of 200,000 were sent In three weeks by ene dealer, and on one day 30,000 were sent by the •am* person. They are sent principally to Rbelms, Nancy, and Paris. A thousand frogs fetch 13 francs (10s Sd.l, and weigh 50 kils., or about one cwt. No duty is charged by the French Custom-house on frogs. At Kheims, 25 pairs of frogs' legs can be bought for GO centimes. The autumn anil spring are the best times of the year for frogs.

COMPOSITIONS FOB THE BOTTOMS OF VESSELS.—Mr. Glbbs, of Milton-street, City, proposes to coat the bottoms of iron vessels, to preserve them from fooling by the employment of a mixture of tallow, or other greasy or fatty matters and resin, to which Is added a saturated solution ef sulphur (in caustic or carbonate alkali or lime). In which copper has also been previously dissolved. The compositions are prepared by the agency of heat or otherwise, and they are to be applied hot to the ships or vessels by means of a brush. The compositiox is prepared by taking sulphur, such aa is known in commerce under that term in the proportion of 171b. to 521b. of pota-h leya of 3&deg. strength, and lib. of piecea or plates of copper, which are all mixed together in a vessel heated by steam or otherwise, the heat being kept up until the sulphur and copper ore thoroughly dissolved. In another vesael, heated in like manner aa aforesaid, is mixed tallow in the proportion of 7Jcwt, to ljcwt. of resin or resin oil, and the hcatia to be kept up until the mixture ia well liquefied. The two mixtures thus separately prepared are to be mixed together and thoroughly amalgamated by agitation in the heating vessels.

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A11 cheque* and Poat Office Orders to b* made pay. able to J. Passmobe Edward*.

I would have every one write what he knows, .in eV aa much aa he knows, but no more; and that not In this only, but in all other subjects: For such a person may have some particular knowledge and experience of the nature of such a person or saeh a fountain, that, as to other things, knows »• more than what everybody does, and yet to keep a shatter wit** this little pittance of his, will undertake to write the) whole body of physlcks: a vice from wkenee great inconveniences derive their original.—Jsfonfo^M'j*. Euayi.

THE TELESCOPE, Ac.

Sir,—" J.T." p. 640 saya : "it require* at least *tf (meaning. I assume a film refractor to see the Satellites of Uranus" Will he kindly tell me where there is s 71n. in England which will show the two brighter ones; or an 8in. that will exhibit all four of the Moons 1 The possessors of such lnatrumenta shou If they be readers of The Enclihh Mechanic, in t interests of science publish the names and addresses of their makers in your columns.

In answer to " Jupiter," (2<>39), p. 642, th* cheapest form of Equatoreal with which I am acquainted" is the Invention of a Mr. Vallance, and Is described witb> engravings, at p. 394 of your secoiud volume, (No. 52). A much more elaborate one is figured on p. 284, Vol. 8 of The English Mkchanic, (no 195); and thislatter possesses tho advantage of adjustability to different latitudea; or Is of the form known as the"universal equatoreal. The tripod stand shown in your eDgraving. though, should be carefully eschcwecX as vibration, greater or less, will take place with tho best possible wooden legged support A brick or stone pier is the only legitimate one. Home and Thornthwalte, the optloiaus, of Sewgate-street Lonand efficient mounting on this prln

don, make a cheap and ciple. If, however, "Jupiter

seeks excellence and

not merely low cost, I have no hesitation in saying that the most perfect form ef rDeuatorealextaot is that of Cooke and Sons, of York. A Dallmeyer object glass on a Cooke mounting would be the very luxury ol observational astronomy.

I am unaware that 1 ever promised to do moretnnn, examine the region round about MixsrKiid Alcor. I am very certain that I never set myself the dreary and difficult task of gauging the msgnltude of ererj point of light I might pick up. However, as a matter ef fact, the weather has been so wretchedly had lor the last month or two, as quite to put a stop to mere stargazing. If your correspondent will ensure me a night at once moonlesB and cloudless, and state in your columnB fcr what date he has secured it, I WUr devote that niirht to his interests. ■, ,. t

AfrofM of the 3in. tele-cope which showed Jupiter's Satellites before sunset on the 26th of last February, I may observe that ita definition must be remarkably sharp, as must also your correspondent'a eye; and further, that the air must have b en exceptionally clear on the occasion to which he refeia. The Instrument of which ho speaks could show Polaris (a 2nd roagnJtune star) at;2 p.m. on a sunny day. were the air tolerably free from vapour, but thrn. as he knows, tnsj Pole Starisinthe least illuminated part of the sky. I .question extremely if he coul.l pick up a 3rd magnitude star in any portion ol the Heavens under the; circumstances specified.

The "Irish Mechanic" (208). p. 660, will find that 2iu. is the.miuimum aperture of a telescope with whlcft he can hope to do any good whatever in astronomical observation; and tip range of this is very limitoji Indeed.

"A Poor Lad "(2140), p. 21, should, by all mean*, expend his £2 in the purchase ofa Sin. speculum. Assuming it to be properly parebolieed, it would b*i equal in separating power to an achromatic of th* same aperture, i.c, would divide doubls stars 0*8T apart; while, according to Browning's formula, its, light-grasping power would be equal to that of an object glass of about 4-2iq. in diameter. A 2}in. object gloss could not possibly divorce anything closer than a pair of stars 2-'5 distant from each other, and *>. standard 11th magnitude star would baits minimnnv. visible, while with*Sin. speculum your correspondent would see those of the 12 3 magnitude.

"H. F."(2l99),p. 22, should read Vole. VIII. an* IX. of the Km;i.on Mechanic.

"Ethardo" (2221), same page, h« only to push in the eye tube ofj any telescope sufficiently beyond tho point ot adjustment for ordinary sight, to anlt hi* own. .

If " Neptnne " (2224), haa been a subscriber to the> English Mechanic for any length of time, he must know that I have, over and over again, declined tosolve Equations, or, in fact, to execute any merelymechanical computation, or " do sums" in any shape? or way. But, even If I made an exception in his favour, I could not do It with the data which ha gives. There Is not a single syllable about the refractive or dispersive indices of the crown and flint ?Issues that he proposes to use. Tho sole Item of inormation is confined to the assertion that be menus to use a crown lens of 6ft. lOln. focal length. Now, If be> has got hold of two of Chance's discs of glass, of which' the dispersive powers are, probably, some*here about ■055 for the Hint, and -039 for the crown, I do not sea

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IMPRECATORY AND EXPLANATORY.

Urn —Boswell records of Doctor Johnson that "A

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ordered to make a map of London from them ; and
whether he would he contented with them en the
ground that tbey might possibly be useful in ao re-
mote a contingency? ,

There is a popular superstition that the materials
forthe useol thecomputers of the "Nautical Almanac
are supplied by the Koyal Observatory at Greenwich.
Is the theory of the Jovian System beneath the nolle,
of that August Establishment; or do the Heads of t
consider that(»o the servaats say) "it !» net their
Place " to help to r.move, what I must persist in
considering to be. ablet from our National knliem.rla?
I fear that We have bare only another lllu«tratlon
ot the truth of t»e old pr.verb, that, " What la every-
body's bu«lnes«,is nobody's business."

I suspect however, that, after all. It Is merely
a question of money. Tables are a.m.what .ostly,
and until publtc optelon is hroueht to bear very
much more directly upon the beaa. »f the Stat,
than It 1s now, there will always be great diffi-
culty In getting pecuniary aid from the Exchequer-
for anything, the us. of which doea noi appeal
Immtdfately to thejofflclal mind. As far as the
Immediate object of this dlton.»lon Is concerned,
though the abolition of a single embassy to one or
th. beggarly Hula German Courts would pay for the
tables Wndwl at once. Th. first year's salaries of
th. wellbr.il hangers on who Idle there at the Public
Cost would more than suffice to secure us trustworthy
data'to eliminate a glaring disfigurement from what
is,otherwise, a perfect marvel ef aeeuraey.
A Fellow Of The Rotal As-rlWfrOincAt. Society.

the subject, from some good treatise, such aa " Her— schel on the Telescope p" as his queationa at present evidently show he needs more information than spaco will permit in the pages of the Mi c Ham... for although four words would suffice to answi r his qneotloha, I fenr the lniermation would not be of much

... r w c tO him. W. rcBKISB. _

THE SUPPO 8ED PLANET, YULOAN. Sib.—Some or yonr readers may be interested in learning that another systematic search for the auspeeu <! Intra-mercurlal 'plaint, Yulcan. commenced, on March 20. Thire will he about twenty-five gentlemen engaged in the affair, mnny ofwhumarelnthepoaeesslnn oi very powerful telescope.. The observation, will extend over the period fr»m March 20 to April 10. During this time the solar diacs will be continually under observation when risible. Th. following are the names of observere, together with a descriptionof the telescope, employed and the times of observation :—

THE BELTS OF JUPITER, Ac.
Srn,—Many thanks to Mr. Praetor and "V.R.A.S.,"
for their answers to my question on the above subject,
and particularly toMr Denning, whose concise atifl ap-
propriate letter (page 043), is all that could bo desired.
1 regrot that I eann.t aay so much for Mr. Grover'a
letter con the aame psgei, and had he given the same
attention to my remark, that he supposes I have
failed to give to Mr. Browning'a remarks in the
Student. Mr. G 'a letter might have been more to
the point; as It is, he makes the question (If I under-
stand him rightly) a question of cAromaficiiy, instead
of one of nlatirt luminoriiy. In my letter 1 purposely
avoided the word cofour, to prevent mUcouceptlon,
but aa Mr. Grover'a letter la almost exclusively on
colour, it falls, so far. to touch the au.sti.in. Lest, how-
ever his remarks should be considered by some of yonr
readers to be a satisfactory a.swer to the question In
hand, I cannot allow them to pass unnoticed. In the
first place, there can bo no doubt that large apertures,
combined with high powers, are necessary to bring out
delicate gradations of colour or chromatlcity -, but the

LIGHT.

BVr,—The following table will probably meet the requlrementa of Mr. A.J. Smart (SOW) p. 642.

•' lenorance, Madam, pure ignorance. Slay mSwedto vindicate myself in the same way with reference to the eba.gc brought against me on p. rv E'eTMomicron"of Joking at his expense? D Arrest ., ionret I did know, the name of the computer of It. XDhVm.rto 1 did not know. I was thankful to »f.mTMron" for the offer of what I know would be B.eiul to a considerable number of observers, and I Jo^lc 'what 1 believed to be; hi. apeUing on trust.

ifTrAC ifUr hl'Ary>«-*

It It will afford Mr. Proctor the very smallest satisfaction 1 will confrisibat I think it extremelyllkely that I was wrong In mv coajecture aetothe possible node ofacfioaef an object (.-lass of large aperture In rendering 5«lar detail visible ; but at the same lime I must as.trre blm that It was not to escape making this admission that I terminated the discussion. as far as I was concerned. To render myself amenable to the charge of ceasing to argue when I find argument going against me, I ought, as It seems to me, to continue to arras when I am on the winning side; win raft ■ it is noiorloua te every one of my brother readers oi the Shclish Mechanic, that 1 have eschewed discussion *'• mifio on every moot point whatever. I have In the outset aaid my aay, and if ever a second letter from me has appeared on the same subject, it has been merely explanatory, or to remove some misconception 4>f my meaning. 1 suppose that there are very few ■men of science who would value, or defer to, Mr. Froetor'a opii.ion more than 1 should j but even tmch deference and appreciation would not prompt me to crumple np a polemical pewter pot merely that I

ml^ht flgnrc among Mr. Proctor's "strong meiK" \£m„7j; I "imagine, doea not apply lo the broad

Bellow or The Royal Astronomical Society. mi "^ rf. ^w ^ >hadf Al an fnltlince 0I th,8i i

may mention that when using a Sin refractor, by
Wray. simultaneously with my Bin. reflector, the dark
bells have appeared much darker with 3ln. than with
91n. aperture, notwithstanding the advantage which
tie reflector possessed over the other in point of defi-
nition, and a precisely similar effsct was produced by
diminishing the aperture of the mirror. Just in pro-
portion aa this diluted the brilliancy of the bright
belts, so it rendered the dark belts still darker, and
thus the same relative contrast between the light
and dark belts remuined unchanged, whatever aper-
ture was used; and more than this, when the air baa
been unsteady, I have always found tho belts more

fiositive and conspicuous with a small, than with a
arge aperture. I canaot quite agree with Mr.
Grover'a opinion that no pigments or materials with
which we are acquainted can be made to accurately
represent the differences which do exist: and yet he
has Jnat admitted that "Mr. Browning's drawing Is
too vivid In colour." Perhaps Mr. Grover can throw
a little more light upon this apparent paradox. In the
meanwhile, my humble opinion is, that at the period at
which Mr. Browning'a drawisg was taken, Jupiter
appeared even more vivid than the engraving in the
Student, and 1 cannot.f or my own part, scetho slightest
difficulty In finding pigments that would have exactly
expressed thia brilliancy, without the heaviness and
opacity which now exist in the engraving, and which
fault, I cannot but think, Is more the result of acci-
dent than of any error In tho ori^inaJ drawing. Again,
the final paragraph In Mr. Grover's letter will not bear
criticism, for if delicate gradation of shade require to
be Bo muoh exaggerated to give sufficient Intensity to
the lights, how Is it that artists are able to give in a
painting the relative degrees of light, shade, and per-
spective to tho clouds of a summer skv, and yet pre-
serve throughout such a gl.wlng luminosity as to far
exceed .the brightest tints of the landscape? Unfor-
tunately for the science of aBtrouomy, It would seem
that but few skilful draughtsmen enlist their talents
in her service, and so diflicultles aro made of things
which ought to be comparatively easy; If, however,
we are yet to gain any further knowledge of the phy-
sical constitution of these remote worlds, it can quly
be by scrupulous accuracy iu observation, dcllaeation,
and description, and anything emanating from so
good an authority as Mr. Browning certainly deserves
criticism, and that is far more than can be said of the
miserable atteaupta at planetary drawing which too
often find their way into public journals and treatises
on astronomy. .

In answer to the question by " Iota (page Zi). I

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With reference to the request of " T. A." on p. 068,1
must regret toy Inability to make even a guess worth
listening lo, as to the reason why the more refrangible
frays ot the spectrum are absent at times from the
light ot the sky. 1 have myself noticed when photo-
graphing, that the time of exposure which sufficed ok
one day was wholly Insufficient on another; but, In-
asmuch as this deficiency of actinism seems te follow
no law, 1 was always content to recognise it aa a fact,
without attempting to account for It.
•A Fellow or The Kotal Abtkonohicax Society.

THS "NAUTICAL ALMANAC " AND "F.B.A.S."

8r»,—I freely admit, in limine, the correctne-B of «he ase.rtion of" W. E. P.," p. 14, that, " It is the duty «f computers attached lo that (the "Nautical Almanac') establishment to uae, and not to form tables." The Superintendent ef the "Nautical Almanac,"as such. IB by no means bound to make his own. All he h as to do is to aee that the compu'atlonB executed by the aid ot thoae supplied to him are correct; and that Mr, Biud d.ss this, and doea It with the very utmost efficiency, no one la readier to allow than lam. . The question, however, 1b a little wider than this. 8wely your correspondent does not mean to contend that a mass of observation, of Jupiter's Satellites has not accumulated which wonld hare sufficed, a long time since, to have formed tho basts of tew and improved tables f Suppose that a man had, In the year ISM, gone up the top of S. Paul's Cathedral In Lon4ou, and had taken a aeries of horizontal angles sub«ended by the various visible church spires. Suppose, .further, that year after year he published his results to the decimal of a second of aro, admitting oandidly Kll the time that his theodolite Was Bo abominably ■centered and badly divided that it could not be depended upon to read within 3 degrees 1 Assuming mots, I say, should We not bo a little disposed to laugh »t our hypothetical observer, when, In this Year of <irace 1870, he defended the publication of ouch admittedly erroneous results on the ground that they (night some day prove of the greatest use, in correcting the theory of Instrumental construction and ad| astments? I wonder what Sir Henry James would »»y if be were restricted to anoh materials and

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Th ere are several gentlemen who intend making •olar observations at Irregular times. These include J. Birmingham, Tuam; Henry Tratt, Brighton j Edmund Kelson, London; Cm. H. Chichester, Fartown, Huddenfield; H. M. Whlttcy, Penarth; and A W. Blacklock, of Manchester.

Should a transit of tho euspected planet occur, the position of ita path across iho solar disc, the time occupied in the transit, and its apparent diameter will be carefully noted. If the observer possess micrometric apparatus, measures of its position on the disc will also be made.

The fact that the period of maximum frequency of the solar spots has arrived -will give additional Interest to the observations. I will duly report to you particulars of the result.

William F. Denning, Hon. See. Observing
Astronomical Society.

POISONOUS POSTAGE STAMPS.!

Sir.—With reference to Mr. Denroy, I cannot help thinking that his friend's illness wns not can»edby the composition on the back of the stamps, which consists only of dextrine, or starch-gum ; and so far from being unwholesome, is actually nutritions. It Is far more likely to have been caused by some *»f the red colouring matter, which so easily comes off. In wetting a quantity of stamps, some portion, more or less, would

firobably he swallowed. By the way, do you suppose t Is generally known that peony and twopenny stamps cost more to print than the higher-priced and more elegant varieties, the former being copper-plate printed and the latter surface printed. This seema a curious anomaly. Let me thank several who have answered queries of mine lately, and congratulate you on the improved form of our Mechanic. A. S. C.

may tell him the announcement by Mr. Wrayas to
the separation of the close pair y> Andromeda, by Mr.
Buckingham's refractor, was made in 1 he "Astrono-
mical Register " for September. 1867 (rage 2»li, and aa
he speaks of the glass being "Just finished, the obser-
vation referred to must then have been quite recent.
The central distance between tho components of this
pair is commonly estimated at half a second. 1 he
late Mr. Dawes' measure in 1803, was 0*-59 ; most 00-
oervers agree as to its being rather over half a second,
although Mr. Browning, in his "Plea," says it Is
under half n second. Al any rate, the discrepancies
arc not great. I believe tho two stars still keep about
the samo distance, no widening having been as yet oo-

B0Wita regard to Andrew Johnaon'B inquiries on page fusion."' H ••' Hugo"" wlli'kTnuiy waft a little, he .ball
21,1 should recommend him to read a little more on | have my Ideas upon algebra. L. H. w. B.

EQUATION.

Sir,—"Hugo" rightly complains of my slip in not putting signs, but he has In tho latter part of his letter overlooked the fact tlint I have noi taken the square root, or why do I use the sign *J ?Asan excuse for carelessness I must plead Illness. I Bent a solution to the problem without revision, as I could scarcely bear to look at a paper, and hence this con

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TO MILLERS.

Sir,—I am a miller of 24 years' standing, and do not remember tbat I ever met with a scientific correspondence between men of my trade until 1 saw It in your Mechanic.

With your permission I will make a few remarks on some of the assertions of your " dusty " correspondents. I am not too old to learn, and If I get hold of an Idea tbat is erroneous 1 am thankful for correction.

J. Sharp says, in No. 258: "As to stones working well when out of standing balance, it only proves the stones are in running balance." I ask blm kindly to explain, as I hare a i o ion tbat if the face of a pair of stones is true, the bedstone level, the driving irons and pitch of oogs of both driver and pinion correct, a standing and running balance are the same. My stones are balanced by Clarke and Durham's patent balance, and I have invariably found irons, face, Ac, correct, after I had given them a perfect standing balance; the running balance was exactly the same. This I prove not only with a twig or chip, but I place a gas Jet on one side of the stones when running, and go on the opposite side and look between them, when If thero is tbe slightest variation in the face it may be seen.

But I have one pair of stones on faulty universal irons, and on them I find a standing and running balance are not the same; from which I Infer that if the above conditions are observed, a standing balance will suffice. A boy's top, when not truly turned, or with a crooked peg, would scarcely spin upright.

If my memory does not deceive me, I saw, about ten years since, an advertisement of the "patent balance," and to me It conveyed the idea that millers were at a loss to account for their etoues dragging when at work, and the "patent balance" professed to remove the evil. It is true many millers cannot account for it, looking only to the face of tbe stones for the cause, and forgetting Irons, level, cogs, Ac. Tbi* has often come under my notice, and surprised me. When I first saw the above advertisement, I aaid, " If a stone will balanco Btandiug, it will running," and I am still of tbe same opinion, after Beven years' experience with tbe " patent balance."

Probably "Cumberland John'' may glean a hint fram the above.

with reference to Mr Lomax's " Improvements in Driving Millstones " (No. 259), 1 cannot help but think it must bo a great nuisance when taking out of or putting Into gear the stones. I thought the fly-wheel wu intended to keep the engine steady; if not, millers should take the hint, and take care their engine has two cylinders. "Prevention is belter than cure."

A Stohejjak.

Sir,—In answer to G. Scott's queries, I beg to aay that a pair of 4ft. French burr mill-stones iclose), with 12 master furrows 2Jin. behind the centre, and two short furrows t« each, ought to grind 10 stones per hour, going at 130 revolutions per minute. But the quantity depends upon the quality and condition of the wheat, and whether ground M high " or "close," and also If the exhaust is used. With respect to the wire maehlne, tbere ought to be 8 brushes in a 20iu. cylinder going at 400 per minute ; but the number of brushes is only a quesliou. ol speed. I had a win* machine at work about ten years ago; it was Oft. long by20ln. diameter, with 8 brushes ; it had 4ln. to the foot fall (Mime machines have more); it made 400 revolutions per minute, and dressed Osks. of 20 stone each per hour. Tbere were only 3J 6heets of wire for fine flour, the rest was covered for white, grey, and brown sbarps; the bran went through the end.

If Mr. Q. Scott la making improvements In his mill,

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Under tbo microscope it Is seen to consist of oil globules of various sizes floating In a transparent fluid called teruni or whey.

Its composition varies considerably, being Influenced by tbo following circumstances :—The age and condition of the cow, the time and frequency of milking, the quality of the food supplied, the housing, weather, Ac. Into these details it is not necessary to enter. The average per centage composition of pure milk is as follows :—

Water 86, casein S, butter 3,, sugar (lactose) 4J, mineral matter 1. The specific gravity is 1-030.

In the above anal) sis the casein represents the "flesh and force producer," and "force-producers"

which develop heat and mechanical force In the systemconsist of the butter and sugar.

Pew articles of consumption are more "doctored" than milk. The principal addition is water, but as this has the effect of giving a "sky-bluo" tinge anil reducing its flavour, it is necessary to employ correctives in the shape of treacle or sugar for sweetening, soft for flavouring, and annatto lor colouring.

Chalk, starch, sheep's brains, Ac, are said to be also used; but this is very doubtful, as their presence weald be betrayed by their insolubility.

The adulteration with water Is easily shown by the "Gravity Lactometer." This is an instrument (Fig. 1> about Bin. long, and constructed to float at certain points, according to the quality of the milk. Thus themarking on tbe stem denotes as follows:—At W thelactometer floats in water; at M In pure milk; at 3, 2.1, in milk containing respectively oni-fourtn,ont half, and thret-fourths water. A

A uother mode of testing is by ascertaining toe quantity of cream yielded. Fig. 2 represents a glass tube about llln. long, with a foot, and with the two upper Inches graduated. The tube is filled up to v with the suspected article, and placod aside for twe'y« hours. Tbo percentage of cream can then be read on. Although tbe quantity of cream yielded by milk lasubject to much variation, yet we ought not to be content, with less than ten per ceut, from good milk.

Both forms of " testers " can be purchased at a small cost of the operative chemists ana opticians.

The detection of the other adulterants is attended with some difficulty, and involves an acquaintanoewith chemistry and tbe use of the microscope.

Beta.

BUILDING AND STEERING VESSELS. Sir,—Although "Scrutiny's" query regarding sailing vessels, p. 579,Vol. X., was answered by " CapeHorn^' (page 633), yet I think I can offer a few remarks on tue subject which may be of use, if not to "Scrutiny," yet to some of your young readers whomay have a nautical ambition, and find a difficulty such us " Scrutiny "complains of in his younger days, In getting his " lugger" to behave herself on a wind. We must not " despise the day of Bmall things " la mechanics or nautical science any more than in religion, and therefore I will try to help others who are following the path of inquiry as I have done, from sailing models in tbe Serpentine some forty yeare ago, 10 the handling of Her Majesty's ships of larger tonnage than "Cape Horn" speaks of, but in all of which the same principle is involved, viz., tbe centre of effort, due of course to the masting and rig ef the vessel and trim of the sails set; however correctly this problem may be solved theoretically whon the vessel Is upright, all Is thrown out when the pressure of the wiud causes the vessel to heel over, as "Scrutiny " observes, the curve of tbe lee bow being Increased while that at the weather bow is decreased at the float line,causing the vessel to rnsh up into the wind and requiring, as " Cape Horn " states, a good deal of weather helm to check and overcome; this action of " griping " as it is termed, Is common to all rigs and all vessels, from a toy to a first-rate, and it ia the perfection of seamanship to so adjust tbe " centre of effort, or "trim" the sails and hull of tbe vessel as to reduce this action to the correct proportion ; as it is evident that if allowed to exist In exiess, tbe leverage of the rudder surface required to overcome its force will seriously retard the speed and, in a storm, endanner the wringing off the rudder head; while, if it la totally counteracted, so as to cause the vessel to carry (aa it is called by seamen) "a slaek helm," there will be great loss of weatherly qualities and danger resulting from any sudden Increase of wind pressure which would strain the rigging aud sails, and tend to plunge or press the lee bow deeply into a sea

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I cannot quite agree with my nautical brother * Cape Horn" iu bis opinion that *• the cause of vessels not steering straight depends greatly on their build." "No doubt such is the case to a certain degree, nnd the great nautical question still Is, What is the beat nliape lor a mailing vessel? but I maintain that the chief cause of bad steering In any vessel properly loaded to her proper displacement is, first, being "oat of trim/* according to her lines or shape, and. secondly, to the sails being not trimmed to the proper angle, <>r judiciously distributed.

The first Is of most Importance in sailing before tho wind and on a wind.

The second is of most fmporfance on a wind and least before the wind. In large voxels it is the pride of her commander and crew to obtain the perfect trim of the sails, while it is the naval architect's pride to calculate correctly what should be the proper trim of the hull, and when these are happily combined a beautiful result is produced, for a full rigged ship in full sail upon a rvind " is a thing to admire." In sailing models, however, it is far different, as there Is no one on boixd the little craft to attend to these points; and though it may appear childish to take up your Tabled space with such a subject. I maintain that it Is not so. lor the skill of the lad in sailing his little home-made model Is really valuable experience, and may become the means of making him a good seaman In after life, and thus maintain our national fame and prosperity. I would therefore ask you, If possible, to allow me to say a few wordB upon 'Model Ve**el«,"for the benefit of some of yonryonoger subscribers and readers, one of whom asked a question on the subject some time back. All models for actual sailing should be rigged in what is called fore and aft rig-, and have Do square sails, so that should they by accident get on " the other tack," they may not become "hove to," as "Scrutiny" evidently describes from painful experience. Fore and aft rigs require the place of the mast or masts to he very correctly fixed, or they will never sail well; this can be very easily found by floating the model properly ballasted, so as to trim by the stern considerably in some still pood, and there with a slight loDg stick posh against her side till the point is found where she will be pushed bodily away. The mast must be placed on a line with that point, or if two masts, equally distant from that line, and the head sails and bowsprit Blade considerably larger than the proper proportion hi large vessels. The rudder must be fixed in tho middle or amidships, and never moved. The whole attention of the young sailor being directed to the trim of the satis, by which the proper direction to the little craft's course can be obtained to a degree of wonderful accuracy, so that she cau be made to run with the wind, or beam, or " close hauled," at pleasure. It Is not so easy to make models sail more off the wind than ■ a beam," and totally impossible to get them to sail "he fore the wind," which should never be the ambition of a British sailor, any more than it Is that of the British soldier to retreat before an enemy. Many a valuable experiment has been solved by aid of the little sailing model, and i: has often been adelight to me to watch the pretty little craft dancing over the mimic waves In our parks, as it has been to see my ship gallantly breasting the mountain billows of the ocean. Ihe Ancient Mariner,

PARABOLISING SPECULA. But,—I trust that Mr. Furklis will pardon my

replying to a question addressed to him, but which seems to have escaped his notice.

"Novice," p 607. Vol. X, states correctly enoueh that the process of parabollalus; a mirror is usually described as flattening the edge, but that the method described by Mr. Furkias seems to be deepening the centre. The accompanying diagram may perhaps make this matter clear to your correspondent. 1 may inentioa that n precisely similar qu**tlou was recently addressed to me privately by a friend, so that probably the reply will be useful to other readers of the English Mechanic as well as " Novice."

Let A B represent a neeti«n of a speculum of spherical curvature, and let C D, C D, und K F, E F represent parallel rays of light falling upon its concave surface. The rays C D, C h, winch fall near the margin, will have their focus at G, while the rays E F, K F, which fall near the centre, will have their locus at II, farther away from the speculum than G, The process ol parabolisatiou bos for iu object to cause all parallel rays to come to a focus at one point, and In the Instance before us this may be accomplished in two ways. We may work the polisher so us to act more upon the marginal than upon the central parts of the speculum, thereby flattening the edge and lengthening' its focus, or, in other words, bringing out G to H\ or we may

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cause the polisher to act more energetically upon the central parts than upon the margin, thereby shortening the locus of the central parts and bringing back H to G. In practice this method, which was introduced by the Kev. H. Cooper Key, is found to be the most certain to produce the required result.

F.8— It seems to me that Mr. Purkiss Is wrong In placing the adjusting screws inside the cell, where he cannot get at them without pulling the whole affair to pieces. Tin-* arrangement makes it impossible to perfect the adjustment under the eye.

AiiTiuii W. Rlacklock, Newbridge,
Uardgate, Aberdeen.

SEA SICKNESS. Sin,—You lately gave a description of Mr. Bessemer's invention tor preventing sea sickness. I understand that a model has been constructed which is stated to work admirably; but I believe this is like many other Inventions, which work well as models, bur are quite useless when carried out OH a large scale. It is well known that the compass always preserves its level position, although it moves up and down and laterally with the heaving and rolling of the vessel; and it would be precisely the same with Mr. BeBsemer's cabin. He seems also to have forgotten that the centre of gravity of the vessel, which is the point of least motion (on which the cabin is to be suspended), alters with every ton of goods shipped or unshipped,

every ton of coals consumed, and also with the pressure of the wind on the sides ol the vessel ; consequently his cubiu would not prevent sea sickness. The vertical motion of the vessel is the chief cause of sea sickness, which cannot be prevented until something is Invented to avoid this mutiou. T. W. Cowan, C.E., l'h.D.

SELF-CENTERING CHUCK. Sir,—" Marine" is in error in saying that u A. B." has put a left-handed screw Into his chuck on p. G31V vol. A. It ts aright* banded screw, and is quite correctly drawn. I know ol a machine with a screw on each end of the mandrel, and the chucks have to be righthanded at one end and left at the other. J. K 1'.

THE ROAD STEAMER.

Sir,—I send you a cutting from a local paper, giving Uij account of a road steamer, which, if all that is said about it bo true,cannot be too well known. I think 1 have not seen it mentioned in our Mechanic*, therefore, I send you this clipping, which you may think worth reproducing. T. T. M.

Mechanical skill hat for many years past been engaged in trying to make our great servant steam work upon common roads. Two radical difficulties, however, baffled the ingenuity and zeal brought to* bear upon the problem. Ponderous traction-engines were built on various plans, but always with theresuit that the shocks experienced in running over hard roads occasioned coutluual breakages iu the maehiucry. But now the two difficulties have been overcome by the invention of the road steamer. The road steamer can run on any kind of road. It runs over hard roads and paved streets without jolting, over soft roads without sinking, over muddy roads without slipping; nay, it needs no road at all, for it can run with equal ease over grass fields, through ploughed fields, upon ice, through loose sand, and over frozen snow. Though small aud light itself, la climbs the severest gradients and draws euormous> loads. It owes all its faculties and its exemption from the disabilities of other traction-engines to one device, as simple as it is efficacious. The wheels, which are of great width, arc surrounded by tires of vulcanised india-rubber. These thick ba» ds of indiarubber enable the road steamer to float over the surfnco of the ground without the slightest damage to> the road, while they likewise protect the mrcbiuery from all concussion. The intervention of the elastic tiros between the wheel and the road aets, in fact, in tho same way aB if the engine were running over a tramway of indiarubber. Mr. R. W. Thomson, C.E., of Edinburgh, the inventor of the road steamer, having experienced much annoyance from the defects of traction-engines, and iiudi'ug none able to do worlt for which Tie required them, conceived and carried out the idea of providing the wheels of astcam-engiue to run on common roads with india-rubber tires of immense thickness. When the first patent rood steamer was tried, some two years ago, its success was complete, and far exceeded the expectations and hopes of the inventor. Since then he has been engaged in building numbers of these engiues to send to all parts of tho world, and the record of some of their performances, iu the presence of engineers, agriculturists, and other practical men, will doubtless be found interesting. A three-horse power engiue drew a boiler weighing 13 tons up an incline of 1 in 12, the grouLd being so slippery at the time from frost that horses could not keep their feet. The engine was run through a grass field without leaving a track, ana again through afield covered to a depth of two feet

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