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A drum is fixed upon the axis of the pulley at the edge of
the table; over this drum is stretched a sheet of paper, and,
during the motion of the pulley, a peucil whose point is
pressed against the surface of paper is caused to move with
uniform velocity parallel to the axis of the drum.

As the pencil has thus two simultaneous motions,
in directions at right angles to each other, a curve
must be the result, and if this curve be a parabola, it
is easily shown that the acceleration of a point on the
drum, and therefore of the sliding body (since the cord
moves with the same velocity as a point on the drum), must
be constant, and consequently that friction is inde pendent of
the velocity; for if the motion of the sliding body be uniformly
accelerated it must be acted upon by a
A
C
constant force, and therefore the friction
cannot vary with the velocity.

FIC.I

B

was

In Fig. 11, let A B E be a portion of the parabolic are described (for in the experiments a parabolic are clearly obtained), whose axis, A D, is at E right angles to the axis of the pulley, let B be the position of the pencil's point after the interval of time from the commencement of the motion, draw B C perpendicular to A C, a tangent at known property of the parabola, if p

A, then, by the well
be the latus rectum

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but
p

AC
BC, but if be the uniform motion given, to
P
12 82

P

is constant, hence B C bears a constant ration to 2, or
the spaces traversed by a point parallel to the axis A D of
the parabola vary as the squares of the times of traversing
them; but 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 f be the dynamical measure of this acceleration in feet
per second, and we have C B = 2, by the relation
12 v2
therefore f
between space and time, also C B = —

p

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P

include every circumstance of the case. As friction considered as a resistance constantly retarding the 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 and axle friction, which together may be taken to be some definite proportion of the weight of the carriages. The resistance of friction on roads and railways shall however receive a separate account hereafter.

The following is the problem :

A body having descended one incline of given length and inclination, is stopped on its descent or ascent) on another also of given length and inclination. To find the friction which stops the body.

a

FIG.12

a

h

In Fig. 12, let the body start from a, and, having descended a b, let it come to rest at d, a point on the incline be. Let B and B be the inclinations of a band be to the horizon. Now if W be the weight of the body, W (sin. Bμ cos. B) as has been before explained, is the force which urges the body down the plane, or, dividing by M, the mass of the body, we have, since W= Mg, g (sin B-u cos. B) for the dynamical acceleration of the downward motion, therefore, if v be the velocity of the body on arriving at 6, and if s be the length of a b, we have 2 2g (sin. B-u cos. B) s, by the relation between velocity and space described. Now if we suppose the body to pass from ab to be without loss of velolocity, we have its velocity on starting from bequal to r, and by the nature of the question, this velocity is destroyed by friction when the body arrives at d, therefore the friction, or μ W cos. B', is greater than W sin. B', or the component of the weight along the plane be. The dynamical retardation of cos. B'sin. 3), and the motion along be is therefore gu the relation between the final velocity (in this case o) and the space described when the body has an initial velocity, we must have o = x2 - 2g (μ cos. B'-sin. B') s'

He commenced by giving some farther particulars relative to
the artificial modes of producing ice. The materials suitable
for abstracting heat from water in sufficient quantities to be
available for reducing temperature were, líquid ammonia,
afrobal, and ether. The substance hitherto used for pro-
der cold had been almost entirely ether vapour, with the
exception of the expansion of air, To produce a lb. of ice, we
required to take out of water at a temperature of 60° a quan-
tity of heat represented by 142 heat units, and to cool the
water from the temperature of 60° to that of 32°. Speaking
of this in larger quantities, the lecturer said that 2848)b. of
ether required to be vaporised in order to convert one ton of
water at 60 into ice at 320. Although there was a great
difference between the quantity of those materials which
were requisite to produce this change, experience went to
show that the solution of ammonia was the most efficient
material to be used for absorbing heat from water to convert
it into ice. Still, ether had the advantage of being suscep-
tible to vaporisation at low temperatures. Neither water nor
alcohol would vaporise at a low temperature with anything
Eike that degree of rapidity which would suffice for artificial D
cooling. The doctor then described the way in which ether
was used. It might appear remarkable that fuel should
be used for producing cold, but the reason was, that the rate
of evaporisation that corresponded to the tension of ether
would be too large to produce any appreciable cooling, or
sufficient cooling of water to produce ice. Therefore, the
evaporisation of ether had to be facilitated by mechanical
meana. With the aid of a diagram the lecturer explained the
whole of this process, and then spoke of the ether apparatus.
A machine capable of producing five tons of ice in the space
of 24 hours consumed a quantity of fuel which was in the
proportion of llb. of fuel to 3lb. of ice produced. The
tension of vapour varied according to temperature, and that the pencil, A Ctr, therefore A C2 and B C =
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 had to be consumed in making ice by means
of ether. Ammonia, at an ordinary temperature, was a
gaseous substance, requiring a pressure of about 150lb. on
the square inch to reduce and maintain it in a liquid form. If
we used ammonia to produce ice, it must be at the expendi-
ture of a considerable amount of power and of fuel. When
we remembered that in the steam engine there was not,
generally speaking, more than one-tenth of the heat produced
by coal burnt which became affected by mechanical force, it
must be evident that this mode of working ammonia must be
But, on the
very costly; and so it had been found to be.
other hand, 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
In experiments such as the above, where acceleration takes
faculty of ammonia for being absorbed into water, and this place, it is clear that the descending weight exceeds the
ebaracteristic had been taken advantage of in working the friction by a certain quantity; we must therefore find an ex-
ammonia machine, as he showed by reference to two diagrams pression for u in this case in terms of known quantities. Let Q
of ammonia machines used in the production of cold. In both be the sliding body drawn along by a descending weight P.
a strong solution of ammonia was separated, and used over then we have the weight P+Q set in motion by the force P
again; but the special advantage of one of them was that
HQ, since the moving force is P lessened by the friction
liquid ammonia was obtained much more free from water than
in the other. The expansion of air had also been applied to, and if f be the acceleration, as found above, we must
this purpose; but this was so expensive a way of producing have by the laws of motion, P+Q : P − μ Q : : g : ƒ †,
refrigeration, as almost to place its use out of the question. where g is the dynamical force of gravity expressed in feet per
Artificial refrigerators were now in use, and he had only second.
P(g-f)-Qr
lately been informed that in the breweries of London there
were cooling machines working which were capable of cooling
as much as nine hundred barrels an hour to the extent of 10
degrees, which was equivalent to the cooling of 324,000 gal-
lons an hour, and that quantity cooled 10 degrees was equi-
valent to the melting of 10 tons of ice an hour. At this
point Dr. Paul entered upon that part of his subject which
should have commenced his lecture. He began by showing
the uses of fuel for the production of high temperatures, and
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 heat.
This was espe-
cially important in the use of fuel in such operations as the
manufacture of iron. The specific heat of almost all sub-
stances increased as the temperature increased. In the case
of iron it was more than double at that high temperature
required to be maintained in the puddling furnace. The con-
sumption 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 represented in the
actual work done. By this 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 circumstances, was indispensable. Speaking
of gas, the lecturer said that the specific heat requisite to
raise the temperature of 131b. of gas 1 degree was about
three heat unita. In the furnace three-eighths of the heat
was used, and five-eighths passed off in waste; and this pro-
portion applied to glass-making and to all operations neces-
sary to be conducted at a high temperature. The higher the Substituting this value for f in the above value for μ we have The reader may verify the above result in the case of three
temperature produced, the greater the amount of free heat
there was available for 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 blast furnace. By means of a dia-
gram he showed the difference between ordinary combustion
and that taking place in this furnace. In the smelting fur-
nace, carbonic acid was converted into carbonic oxide, which
was combustible, and which furnished heat and 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

e causes which operated in the case of the puddling fur-
nace. There was really 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 as to obviate
this waste consisted in burning it after having inverted it
into a gaseous form, as in the case of the production of gas.

FRICTION.-VI.

BY C. DRAPER, A.B., L.C.E.

(Continued from page 601, Vol. X.)

WHEN speaking of the tribometer, or apparatus for investigating the laws of friction, it was not shown how the law which declares friction to be independent of the velocity was verified; indeed the experiments, as then described, serve only to prove the laws of the friction of rest.

This law can be proved in the following manner.

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v2 = 2g (μ cos. B' — sin. B') s'. Equating this value of to that found above, and dividing across by 2g, we have ssin. B-sμ cos. B = s' μ còs. B's' sin. Bß' Draw now the horizontal lines b c and d h, and put a c b h = c, c b = 1, and hd = x'; therefore sin. B cos. B cos. B' με, μ μ r, and s' sin. B' =, therefore - 2=

εμ

=

$' μ

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=
μ
x + x'

If be be inclined upwards, we have

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μ

=

x + x'

Or, if be be horizontal,

μ

x + x'

Example 10-A body slides down an inclined plane the height of which is 12ft. and length of base 20ft., and slides along a horizontal plane at the bottom for a distance of 52ft. Find the co-efficient of the friction which stopped the motion, on the supposition that the body passes from one plane to the other without loss of velocity. 12, x = 20, and x' = 52 12

Here

1

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=

20 + 52

6

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From this last formula it will be seen that we can determine the coefficient of friction by observing the time the weight takes 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 its axis. They are therefore given to indicate merely the manner of experimenting, as the formulæ actually employed should

*Or, to speak more correctly, the pencil has one motion, and the paper has another; but the result is the same as if the pencil had the combined motion of the two relatively to a fixed point on the paper.

+ 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.

scended, by the total horizontal distance travelled over."
The above is also true for any curved line, as the planes
may be conceived to be so small that in continuation they do
not differ from one continued curved line.

(To be continued.)

THE ACTION OF MAGNETISM UPON LIGHT. FATHER SECCHI, the great Italian physicist, has recently laid before the French Academy of Sciences an account of some experiments he has been making conjointly with Professor Provenzali, upon the modifications produced by magnetism upon the light emitted by the rarefied gases. In theire xperiments they employed a large Bunsen's battery, capable of giving sparks eight centimetres in length from the induction coil. The gases were confined in Geissler's tubes.

The results of their observations have been classed under two heads:

1st. Whenever a tube of wide bore is introduced between or brought into the neighbourhood of the poles of the electromagnet, it is found that the diffused or stratified light which fills the tube when a current is passed through becomes repelled by the poles, and is condensed on the opposite side of the tube, where it appears as a bright streak instead of a Juminous glow. Indeed, the effect is as though the gas itself were really displaced, and accumulated in the more distant part of the tube. Father Secchi considers that the appearance presents a great similarity to the movements of the streamers in the Aurora Borealis

2nd. The condensed gas gives a well-defined and brilliant spectrum, and when the current is of sufficient intensity and the gas is one, such as chlorine or nitrogen, which gives a double spectrum, both spectra may be observed, the one in that part of the tube opposite to the magnetic poles being more intense than that in the main body of the tube. It is considered as proved that the double spectrum is due to a difference of temperature, and not to impurity of the gas, as was supposed by M. Dubrumfaut. A very striking result was presented by the hydrogen 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 1860. The experimentalists think this effect partially due to the decomposition of the glass under the great heat of the current. Father Secchi suggests that a simple solution of the phenomena observed may be summed up thus. Magnetism acts as though it contracted the bore of the tubes, and by producing an elevated temperature causes the gas to give a spectru 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 says it is impossible to observe these phenomena without comparing them 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 1856 to Faraday. We produced the vacuum in an ordinary barometer tube, 36in. long, in the top of which a platinum wire was soldered, the mercury serving as the other conductor from a Rhumkorf. Faraday exhibited the experi ment at a subsequent Friday evening's meeting at the Royal Institution, while lecturing on the induction coll-Ed. E. T and R. R.]-Electric Telegraph and Railway Review.

ASTRONOMICAL NOTES FOR APRIL. BY A FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY. THE Right Ascension of the Sun on the 1st of April will be ob. 42m. 31s., and his North Declination 4° 34' 30". He will, therefore, be close to 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. 568. 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 be

Venus is a morning Star during the entire month'
Southing at 9h. 31m. a.m. on the 1st, and at 9h. 5m, a.m.
on the 30th. She is still a fine telescopic object, albeit
the difference in the thickness of her crescent is very
marked when contrasted with the extraordinary
tenuity which it exhibited in February, Mars is still
too close to the Sun to be visible. Jupiter is now
visibly creeping towards the West. It is on the
Meridian at 2h. 38m, on the 1st, and at 1h. 10m. in the
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. Saturn is
a morning Star until the 19th, after which he rises
before midnight: he is, however, close to the horizon,
and wretchedly situated for observation. He is on
the confines of Sagittarius. Uranus may still be
observed before midnight; his position is Gemini
has not sensibly altered from that laid down in our
little map on p. 470. Neptune is now invisible, in fact
he is 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 a transit of the 1st
Satellite. It will enter on to Jupiter's disc at 9h. 7m.
On the 2nd the 1st Satellite will re-appear from Eclipse
at 9h. 29m. 328., but Jupiter will be very low down.
The shadow of the same Satellite will pass off the
Planet's limb at 6h. 41m. 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 from Eclipse at 7h. 19m, 598, on the evening of
of the Shadow of the 2nd Satiliite will take place at
the 4th from being observed. On the 5th the Ingress

LETTERS TO THE EDITOR.

(We do not hold ourselves responsible for the opinions of our correspondents. The EDITOR respectfully requests that all communications should be draw up as briefly as possible.]

All communications should be addressed to the EDITOR of the ENGLISH MECHANIC, 31, Tavistock street, Covent Garden, W.C.

All eheques and Post Office Orders to be made pay able to J. PASSMORE EDWARDS.

I would have every one write what he knows, and as much as 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 such a fountain, that, as to other things, knows no more than what everybody does, and yet to keep a slutter with this little pittance of his, will undertake to write the conveniences whole body of physicks: a vice from whenee great, inderive their original. - Montaigne's Essays.

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THE TELESCOPE, &c.

SIR,-"J.T." (meaning. I assume a 6in) refracter to see the Satellites p. 640 says: "it requires at least a in. in England which will show the two brighter of Uranus Will he kindly tell me where there is a ones; or an Sin. that will exhibit all four of these Moons? The possessors of such instruments should, if they be readers of THE ENGLISH MECHANIC, in the interests of science publish the names and addresses of their makers in your columns.

In answer to "Jupiter," (2039),

7h, 30m. The Satellite itself will pass off at the opposite form of Equatoreal with which, the cheapest

limb at Sh. 20m., and its shadow may probably be seen to follow it at 9h. 55m., but Jupiter will then be on the Point of setting. On the 9th, Satellite 1 will be occulted at 8h. 30m. On the 10th the same Satellite will leave Jupiter's disc at 7h. 52m., to be followed by its shadow at 8h. 36m. On the 11th the 3rd Satellite will re-appear from occultation at 8h. 42m., only to be Eclipsed by Jupiter's Shadow at 9h. 29m. 268. This is only some 18 minutes, though, before he will set. The 2nd Satellite will enter on to the Planet's limb at 8h. 42m. on the night of the 12th. On the Evening of the 17th there come subtractive, and on the 30th, 2m. 55s. must be will be a transit of the 1st Satellite. It will pass on to taken from the instant of apparent noon to obtain the Jupiter's disc at 7h.40m., and its shadow will come after time, which a properly regulated clock ought to show. it at 8h. 18m. The first Satellite may also be seen in the The Sun is now longer above the horizon, in these lati- twilight to re-appear from Eclipse at 7h. 49m. 168. on tudes, than he is beneath it; the length of the day at the evening of the 18th. Five minutes before Jupiter sets the beginning of April being 12h. 33m., and on the last-viz., at 9h. 15m. 338, on the night of the 21st, the 2nd day of the month 14h. 42m.

There would seem to be some reason to expect Shoot ing Stars in April, during two intervals; 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.

Satellite will re-appear from Eclipse. The 2nd SatelThe Moon enters her First quarter at 25 and a half lite will be occulted at 8h, 34m. on the 28th, or on the minutes past 4 a.m. on the 9th; is Full at 10h. 26m, on succeeding Evening, the 29th, the ingress of the shadow the night of the 15th, when, as she will be in Perigee, of the 3rd will take place at 7h. 26m.: the egress of or at her nearest to the Earth, about half an hour the Satellite itself occurring at 7h. 56m. We should afterwards, a high tide may be looked for: She will perhaps add, that as many of the phenomena, which we enter her Last Quarter at 4h. 25m. on the afternoon of have included in the foregoing list will take place in the 22nd: and be New at 6h. 37m. on that of the 30th. strong twilight, or when the planet is almost close to The Moon will be in conjunction with Jupiter at 49 the horizon, it will require a good telescope to observe minutes past 10 on the morning of the 4th: in conjunc-them: even assuming, what is problematical, that such tion with Uranus at 2h. 38m. in the early morning an instrument will render all of them visible. of the ninth in conjunction with Saturn at 3h, 40m. a.m. on the 20th: in conjunction with Venus at 9h. 53m. am. on the 26th: and finally with Mars at 4h. 10m. 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 1h. 47m. in the early morning of the 9th, 56 Geminorum will disappear at the Moon's dark limb; and re-appear at her bright limb at 1h. 59m. At 2h. 15m. a.m. on the 12th, 34 Leonis will disappear at the dark limb; to re-appear again at the bright one at 4 minutes past 3. At 48 minutes after midnight on the 13th, b Virginis will be occulted by the Moon's dark limb, and emerge from her bright limb at 2 minutes past 1. The bright limb of the moon will occult Libræ at 12h. 16m. on the night of the 17th, and at 6 minutes past 1 it will re-appear from the dark one. The occultation of Saturn will take place during the early morning of the 20th. The planet will disappear at the bright limb at 2h. 55m., and re-appear at the dark one at 4h. 5m. Lastly, Sagittarii will be occulted by the Moon before she rises after midnight on the 20th; but, after she has risen, will re-appear from her dark limb at 6 minutes past 1. The Moon's age on the 1st, at noon, is 04 days; She is 14 days old at noon on the 2nd, and so on. At 9 o'clock in the evening of the 9th, Libration will bring a portion of the S.E. part of her disc into view; while at midnight on the 21st 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 practically 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.

CONSUMPTION OF FROGS IN FRANCE.-The Echo du Luxembourg states that the exportation of frogs from that country to France has increased considerably by one dealer, and on one day 30,000 were sent by the of late. Upwards of 200,000 were sent in three weeks Nancy, and Paris. A thousand frogs fetch 13 francs same person. They are sent principally to Rheims, (10s. 5d.), and weigh 50 kils., or about one cwt. No duty is charged by the French Custom-house on frogs. centimes. The autumn and spring are the best times At Rheims, 25 pairs of frogs' legs can be bought for 60 of the year for frogs.

COMPOSITIONS VESSELS.-Mr. FOR THE BOTTOMS OF proposes to coat the bottoms of iron vessels, to Gibbs, of Milton-street, City, preserve them from fouling by the employment of a and resin, to which is added a saturated solution of mixture of tallow, or other greasy or fatty matters 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 or vessels by means of a brush. The composition is otherwise, and they are to be applied hot to the ships prepared by taking sulphur, such as is known in commerce under that term in the proportion of 171b. to 521b. of potash leys of 35deg. strength, and 1lb. of pieces or plates of copper, which are all mixed heat being kept up until the sulphur and copper are together in a vessel heated by steam or otherwise, the thoroughly dissolved. like manner as aforesaid, is mixed tallow in the proIn another vessel, heated in portion of 7cwt. to 1hcwt. of resin or resin oil, and the heat is to be kept up until the mixture is well liquefied. The two mixtures thus separately prepared by agitation in the heating vessels. are to be mixed together and thoroughly amalgamated

the invention of a Mr. Vallance, and is described with am acquainted is much more elaborate one is figured on p. 284, Vol. engravings, at p. 394 of your second volume, (No. 52). 8 of THE ENGLISH MECHANIC, (No. 195); and this latter possesses the advantage of adjustability to different latitudes; or is of the form known as the your engraving, though, should be carefully eschewed "universal equatoreal." The tripod stand shown in as vibration, greater or less, will take place with the best possible wooden legged support. stone pier is the only legitimate one. Horne and A brick or don, make a cheap and efficient mounting on this prinThornthwaite, the opticians, of Newgate-street, Lonciple. If, however, "Jupiter seeks excellence and that the most perfect form of Equatoresl extant is that not merely low cost, I have no hesitation in saying of Cooke and Sons, of York. A Dallmeyer object glass observational astronomy. on a Cooke mounting would be the very luxury of

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examine the region round about Mizar and Alcor. I am unaware that I ever promised to do more than and difficult task of gauging the magnitude of every I am very certain that I never set myself the dreary point of light I might pick up. However, as a matter of fact, the weather has been so wretchedly bad for the last month or two, as quite to put a stop to mere star gazing. If your correspondent will ensure me a your columns for what date he has secured it, I will night at once moonless and cloudless, and state in devote that night to his interests.

Satellites before sunset on the 26th of last February Apropos of the 3in. telescope which showed Jupiter's sharp, as must also your correspondent's eye; and I may observe that its definition must be remarkably further, that the air must have b en exceptionally clear on the occasion to which he refers. The instrument tolerably free from vapour, but then, as he knows, the of which he speaks could show Polaris (a 2nd magnitune star) at 2 p.m. on a sunny day, were the air Pole Star is in the least illuminated part of the sky. question extremely if he could pick up a 3rd magnitude star in any portion of the Heavens under the circumstances specified.

The "Irish Mechanic" (208), p. 660, will find that he can hope to do any good whatever in astronomical 2in. is the minimum aperture of a telescope with which observation; and the range of this is very limited indeed.

expend his £2 in the purchase of a sin. speculum. "A Poor Lad" (2140), p. 21, should, by all means, equal in separating power to an achromatic of the Assuming it to be properly parabolised, it would be same aperture, i.e., would divide double stars 0-87 apart; while, according to Browning's formula, its object glass of about 4-2in. in diameter, light-grasping power would be equal to that of an A 2in, object glass could not possibly divorce anything closer standard 11th magnitude star would be its minimum than a pair of stars 2-5 distant from each other, and dent would see those of the 12 3 magnitude. visible, while with a 5in. speculum your correspon

IX. of the ENGLISH MECHANIC. "H. F." (2199), p. 22, should read Vols, VIII. and

the eye tube of any telescope sufficiently beyond the "Ethardo" (2221), same page, has only to push in point of adjustment for ordinary sight, to suit his

own.

ENGLISH MECHANIC for any length of time, he must If "Neptune " (2224), has been a subscriber to the know that I have, over and over again, declined to solve Equations, or, in fact, to execute any merely mechanical computation, or "do sums" in any shape or way. But, even if I made an exception in his gives. There is not a single syllable about the refracfavour, I could not do it with the data which he glasses that he proposes to use. The sole item of intive or dispersive indices of the crown and flint formation is confined to the assertion that he means to use a crown lens of 6ft. 10in. focal length. Now, if be has got hold of two of Chance's discs of glass, of which the dispersive powers are, probably, somewhere about 055 for the flint, and 039 for the crown, I do not see

APRIL 1, 1870.]

ordered to make a map of London from them; and how he can possibly achromatise such crown (assuming its focus to be 6ft. 10in.) without combining it whether he would be contented with them on the with a flint one of about 9ft. 8in. focal length; which ground that they might possibly be useful in so remote a contingency? would cause the joint focus to approach to some 23ft. There is a popular superstition that the materials er äft. longer than he seeks to obtain. This is the result of the very roughest conceivable calculation on for the use of the computers of the "Nautical Almanac" the margin of my writing paper, and has no preten-are supplied by the Royal Observatory at Greenwich. Is the theory of the Jovian System beneath the notice sion to be considered rigidly accurate through. If Neptune" will get "Coddington's Optics," of that August Establishment; or do the Heads of it can use a table of logarithms, and will, first and fore- consider that fas the servants say) "it is not their most, determine the refractive and dispersive powers place" to help to remove, what I must persist in of the materials he proposes to work with, he ought considering to be, a blot from our National Ephemeris? to have no very hard task in getting out the curves for I fear that we have here only another illustration himself. It is not so much a difficult calculation, as a of the truth of the old proverb, that," What is everybody's business, is nobody's business." long and very dreary one. I suspect, however, that, after all, it is merely AFELLOW OF THE ROYAL ASTRONOMICAL SOCIETY. a question of money. Tables are somewhat costly, and until public opinion is brought to bear very much more directly upon the heads of the State than it is now, there will always be great difficulty in getting pecuufary aid from the Exchequer for anything, the use of which does not appeal immediately to the official mind. immediate object of this discussion is concerned. though, the abolition of a single embassy to one of the beggarly little German Courts would pay for the tables demanded at once. The first year's salaries of the well-bred 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 of accuracy. À FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY.

DEPRECATORY AND EXPLANATORY. STB,-Boswell records of Doctor Johnson that "A Tady once asked him how he came to define Pastern the kuce of a horse? Instead of making an elaborate once answered defence, as she expected, he at Ignorance, Madam, pure ignorance." May I be allowed to vindicate myself in the same way with reference to the charge brought against me on p. 655 by Omicron" of joking at his expense? D'Arrest's Comet I did know; the name of the computer of its to Ephemeris I did not know. I was thankful "Omicron" for the offer of what I know would be useful to a considerable number of observers, and I took (what I believed to be; his spelling on trust. Hinc ille lachryme.

If it will afford Mr. Proctor the very smallest satisfaction I will confess that I think it extremely likely that I was wrong in my conjecture as to the possible mode of action of an object glass of large aperture in rendering solar detail visible; but at the same time I must assure him 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 argue when I am on the winning side; wherea it is notorious to every one of my brother readers of the ENGLISH MECHANIC, that I have eschewed discussion ab initio, on every moot point whatever. I have in the outset, said my say, and if ever a second letter from me has appeared on the same subject, it has been merely explanatory, or to remove some misconception of my meaning. I suppose that there are very few men of science who would value, or defer to, Mr. Proctor's opinion more than I should; but even such deference and appreciation would not prompt me to crumple up a polemical pewter pot merely that I might figure among Mr. Proctor's "strong men." A FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY.

LIGHT.

As far as the

THE BELTS OF JUPITER, &c.

SIR, Many thanks to Mr. Proctor and "F.R.A.S.," for their answers to my question on the above subject, and particularly to Mr. Denning, whose concise and appropriate letter (page 655), is all that could be desired.

to

Sir,-The following table will probably meet the gin. aperture, notwithstanding the advantage which requirements of Mr. A. J. Smart (2004) p. 642.

Colours.

Length of
Undula-
tious in
inch.

Number

of Un- Number of Undudulations lations per second. parts of an in an inch

regret that I cannot say so much for Mr. Grover's letter (on the same page), and had he given the same attention to my remarks that he supposes I have failed to give to Mr. Browning's remarks in the Student. Mr. G.'s letter might have been more the point; as it is, he makes the question (if I understand him rightly) a question of chromaticity, instead of one of relative luminosity. In my letter I purposely avoided the word colour, to prevent misconception, but as Mr. Grover's letter is almost exclusively on colour, it fails, so far, to touch the question. Lest, bowever, his remarks should be considered by some of your readers to be a satisfactory answer to the question in hand, I cannot allow them to pass unnoticed. In the first place, there can be no doubt that large apertures, combined with high powers, are necessary to bring out delicate gradations of colour or chromaticity; but the same rule, I imagine, does not apply to the broad masses of light and shade. As an instance of this, I may mention that when using a 3in refractor, by Wray, simultaneously with my 9in. reflector, the dark belts have appeared much darker with 3in. than with the reflector possessed over the other in point of definition, and a precisely similar effect was produced by diminishing the aperture of the mirror. Just in proportion as 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 beits remained unchanged, whatever aperture was used; and more than this, when the air has been unsteady, I have always found the belts more 458,000000, 000000 positive and conspicuous with a small, than with a 477,000000,000000 large aperture. I cannot quite agree with Mr. 506,000000,000000 Grover's opinion that no pigments or materials with 585,000000,000000 which we are acquainted can be made to accurately 577,000000,000000 represent the differences which do exist; and yet he 622,000000,000000 has just admitted that "Mr. Browning's drawing is 658,000000,000000 too vivid in colour." Perhaps Mr. Grover can throw 699.000000,000000 a little more light upon this apparent paradox. In the 727.000000.000000 meanwhile, my humble opinion is, that at the period at which Mr. Browning's drawing was taken, Jupiter With reference to the request of "T.A." on p. 658, I appeared even more vivid than the engraving in the must regret my inability to make even a guess worth Student, and I cannot, for my own part, see the slightest listening to, as to the reason why the more refrangible difficulty in finding pigments that would have exactly rays of the spectrum are absent at times from the expressed this brilliancy, without the heaviness and light of the sky. I have myself noticed when photo-opacity which now exist in the engraving, and which graphing, that the time of exposure which sufficed on fault, I cannot but think, is more the result of accione day was wholly insufficient on another; but, in- dent than of any error in the original drawing. Again, asmuch as this deficiency of actinism seems to follow the final paragraph in Mr. Grover's letter will not bear no law, I was always content to recognise it as a fact, criticism, for if delicate gradation of shade require to without attempting to account for it. be so much exaggerated to give sufficient intensity to A FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY. the lights, how is it that artists are able to give in a painting the relative degrees of light, shade, and per"F.R.A.S.",spective to the clouds of a summer sky, and yet preserve throughout such a glowing luminosity as to far exceed the brightest tints of the landscape? Unfortunately for the science of astronomy, it would seem that but few skilful draughtsmen enlist their talents in her service, and so difficulties are made of things which ought to be comparatively easy; if, however, we are yet to gain any further knowledge of the physical constitution of these remote worlds, it can only be by scrupulous accuracy in observacion, delineation, and description, and anything emanating from 80 good an authority as Mr. Browning certainly deserves criticism, and that is far more than can be said of the miserable attempts at planetary drawing which too often find their way into public journals and treatises on astronomy.

Extreme Red
Red

Orange

Yellow

Green

Blue

Indigo

0-0000266 37640 0.0000256 39180 0.0000240 41010 0.0000227 44000 0-0000211 47460 0.0000196 51110 0-0000185 54070

Violet

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Extr. Violet 0 0000167 59750

THE 'TM“ NAUTICAL ALMANAC" AND

SIR-I freely admit, in limine, the correctne-s of the assertion of" W. E. P.," p. 14, that, "It is the duty of computers attached to that (the 'Nautical Almanac") establishment to use, and not to form tables." The Superintendent of the "Nautical Almanac," as such, is by no means bound to make his own. All he has to do is to see that the computations executed by the aid of those supplied to him are correct; and that Mr. Hind dees this, and does it with the very utmost efficiency, no one is readier to allow than I am. The question, however, is a little wider than this, Surely your correspondent does not mean to contend that a mass of observations of Jupiter's Satellites has not accumulated which would have sufficed, a long time since, to have formed the basis of Lew and improved tables? Suppose that a man had, in the year 1820, gone up the top of S. Paul's Cathedral in London, and had taken a series of horizontal angles subtended by the various visible church spires. Suppose, further, that year after year he published his results to the decimal of a second of arc, admitting candidly all the time that his theodolite was so abominably centered and badly divided that it could not be depended upon to read within 3 degrees! Assuming this, I say, should we not be a little disposed to laugh at our hypothetical observer, when, in this Year of Grace 1870, be defended the publication of such admittedly erroneous results on the ground that they might some day prove of the greatest use, in correctIng the theory of instrumental construction and adjustments? I wonder what Sir Henry James would say if he were restricted to such materials and

In answer to the question by "Iota" (page 22), I
may tell him the announcement by Mr. Wray as to
the separation of the close pair y Andromeda, by Mr.
Buckingham's refractor, was made in the "Astrono-
mical Register" for September, 1867 (page 201), and as
he speaks of the glass being "just finished," the obser-
vation referred to must then have been quite recent,
The central distance between the components of this
pair is commonly estimated at half a second.
late Mr. Dawes' measure in 1863, was 0.59; most ob-
servers agree as to its being rather over half a second,
although Mr. Browning, in his "Plea," says it is
under half a second. At any rate, the discrepancies
are not great. I believe the two stars still keep about
the same distance, no widening having been as yet ob-
served.

The

With regard to Andrew Johnson's inquiries on page 21, 1 should recommend him to read a little more on

the subject, from some good treatise, such as "Her schel on the Telescope;' as his questions at presen evidently show he needs more information than space will permit in the pages of the MECHANIC; for al though four words would suffice to answer his quesW. PURKISS. tions, I fear the information would not be of muc service to him.

THE SUPPO SED PLANET, VULCAN. SIR-Some of your readers may be interested i Tearning that another systematic search for the sus pecud intra-mercurial planet, Vulcan, commence on March 20. There will be about twenty-five gentle men engaged in the affair, many of whom are in the pos session of very powerful telescopes. The observationa will extend over the period from March 20 to April 10 During this time the solar discs will be continually under observation when visible. The following are the names of observers, together with a description of the telescopes employed and the times of observation:

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There are several gentlemen who intend making solar observations at irregular times. These include J. Birmingham, Tuam; Henry Pratt, Brighton; Edmund Neison, London; Capt. H. Chichester, Fartown, Huddersfield; H. M. Whittey, Penarth; and W. Blacklock, of Manchester.

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

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. Sec. Observing
Astronomical Society.

It is far more

POISONOUS POSTAGE STAMPS.] SIR,-With reference to Mr. Denroy, I cannot help thinking that his friend's illness was not caused by the composition on the back of the stamps, which consists only of dextrine, or starch-gum; and so far from being unwholesome, is actually nutritious. likely to have been caused by some of the red colouring matter, which so easily comes off. In wetting a quantity of stamps, some portion, more or less, would probably be swallowed. By the way, do you suppose it is generally known that penny 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 seems 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. Č.

EQUATION.

SIR,-"Hugo" rightly complains of my slip in not putting signs, but he has in the latter part of his letter overlooked the fact that I have not taken the ? As an square root, or why do I use the sign excuse for carelessness I must plead illness. I sent a solution to the problem without revision, as I could scarcely bear to look at a paper, and hence this confusion. If " Hugo" will kindly wait a little, he shall C. H. W. B. have my ideas upon algebra.

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TO MILLERS.

SIR,-I am a miller of 24 years' standing, and do not remember that I ever met with a scientific correspondence between men of my trade until I 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 that is erroneous I 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 him kindly to explain, as I have a 10 ion that if the face of a pair of stones is true, the bedstone level, the driving irons and pitch of cogs 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, &c., 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 there is the 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 stones 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 the stones for the cause, and forgetting irons, level, cogs, &c. This has often come under my notice, and surprised me. When I first saw the above advertisement, 1 said, "If a stone will balance standing, it will running," and I am still of the same opinion, after seven years' experience with the "patent balance."

Probably "Cumberland John" may glean a hint from the above.

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

INTERIOR OF PASSENGER CAR.

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Under the microscope it is seen to consist of oil globules of various sizes floating in a transparent fluid called serum or whey.

SIR,-In answer to G. Scott's queries, I beg to say that a pair of 4ft. French burr mill-stones (close), with 12 master furrows 24in. behind the centre, and two short furrows to each, ought to grind 16 stones per hour, going at 130 revolutions per minute. But the quantity depends upon the quality and condition of the wheat, and whether ground "high" or "close," and also if the exhaust is used. With respect to the wire machine, there ought to be 8 brushes in a 20in. cylinder going at 400 per minute; but the number of brushes is only a question of speed. I had a wire machine at work about ten years ago; it was 6ft. long by 20in. diameter, with 8 brushes; it had 4in. to the foot fall (some machines have more); it made 400 revolutions per minute, and dressed 6sks. of 20 stone each per hour. There were only 3 sheets of wire for Water 86, casein 5, butter 3, sugar (lactose) 4, fine flour, the rest was covered for white, grey, and mineral matter 1. The specific gravity is 1-030. brown sharps; the bran went through the end. In the above analysis the casein represents the If Mr. G. Scott is making improvements in his mill," flesh and force producer," and "force-producers"

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

which develop heat and mechanical force in the system consist of the butter and sugar.

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

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

The adulteration with water is easily shown by the "Gravity Lactometer." This is an instrument (Fig. 1) about 6in. long, and constructed to float at certain points, according to the quality of the milk. Thus the marking on the stem denotes as follows:-At W the lactometer floats in water; at M in pure milk; at 3, 2, 1, in milk containing respectively one-fourth, one half, and three-fourths water.

Another mode of testing is by ascertaining the quantity of cream yielded. Fig. 2 represents a glass tube about 11in. long, with a foot, and with the two upper inches graduated. The tube is filled up to 0 with the suspected article, and placed aside for twelve hours. The percentage of cream can then be read off. Although the quantity of cream yielded by milk is subject to much variation, yet we ought not to be content with less than ten per cent, from good milk.

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

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

BETA.

BUILDING AND STEERING VESSELS. SIR,-Although "Scrutiny's" query regarding sailing vessels, p. 579, Vol. X., was answered by" Cape Horn" (page 633), yet I think I can offer a few remarks on the subject which may be of use, if not to "Scrutiny," yet to some of your young readers who may 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 small things" in 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 the Serpentine some forty years ago, to 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., the centre of effort, due of course to the masting and rig of the vessel and trim of the sails set; however correctly this problem may be solved theoretically when the vessel is upright, all is thrown out when the pressure of the wind causes the vessel to heel over, as "Scrutiny" observes, the curve of the lee bow being increased while that at the weather bow is decreased at the float line, causing the vessel to rush 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 firstrate, and it is the perfection of seamanship to so adjust the "centre of effort, or "trim" the sails and hull of the vessel as to reduce this action to the correct proportion; as it is evident that if allowed to exist in excess, the leverage of the rudder surface required to overcome its force will seriously retard the speed and, in a storm, endanger the wringing off the rudder head; while, if it is totally counteracted, so as to cause the vessel to carry (as it is called by seamen) "a slack

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belm," 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.
I cannot quite agree with my nautical brother
Cape Horn" in his opinion that "the cause of ves-
sels not steering straight depends greatly on their
build." No doubt such is the case to a certain degree,
and the great nautical question still is, What is the
best shape for a sailing vessel? but I maintain that
the chief cause of bad steering in any vessel pro-
perly loaded to her proper displacement is, first, being
out of trim," according to her lines or shape, and,
secondly, to the sails being not trimmed to the proper
angle, or judiciously distributed.

The first is of most importance in sailing before the wind and on a wind.

The second is of most importance on a wind and least before the wind. In large vessels 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 wind is a thing to admire." In sailing models, however, it is far different, as there is no one on board the little craft to attend to these points; and though it may appear childish to take up your valued space with such a subject, I maintain that it is not so, for 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 words upon

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ADVANCING THE SHIELD.-INTERIOR OF THE TUNNEL.

replying to a question addressed to him, but which
seems to have escaped his notice.
"Novice," p. 607, Vol. X., states correctly enough that
the process of parabolising a mirror is usually described
as flattening the edge, but that the method described
by Mr. Purkiss seems to be deepening the centre. The
accompanying diagram may perhaps make this matter
clear to your correspondent. I may mention that a
precisely similar question 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 section of a speculum of spherical
curvature, and let CD, CD, and EF, E F represent
parallel rays of light falling upon its concave surface.
The rays CD, CD, which fall near the margin, will
have their focus at G, while the rays E F, E F, which
fall near the centre, will have their focus at H, farther
away from the speculum than G. The process of
parabolisation has for its 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.
may work the polisher so as 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

A

D

B

H

E

We

Model Vessels," for the benefit of some of your younger 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 no 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 be very correctly fixed, or they will never sail well; this can be very easily found by floating the model' properly cause the polisher to act more energetically upon the ballasted, so as to trim by the stern considerably in central parts than upon the margin, thereby shortensome still pond, and there with a slight long sticking the focus of the central parts and bringing back push against her side till the point is found where she H to G. In practice this method, which was introwill be pushed bodily away. The mast must be placed duced by the Rev. H. Cooper Key, is found to be the on a line with that point, or if two masts, equally most certain to produce the required result. distant from that line, and the head sails and bowsprit made considerably larger than the proper proportion in large vessels. The rudder must be fixed in the middle or amidships, and never moved. The whole attention of the young sailor being directed to the trim of the sails, by which the proper direction to the little craft's course can be obtained to a degree of wonderful accuracy, so that she can 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 before 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 it bas often been a delight 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. THE ANCIENT MARINER.

placing the adjusting screws inside the cell, where he
P.S-It seems to me that Mr. Purkiss is wrong in
cannot get at them without pulling the whole affair to
pieces. This arrangement makes it impossible to
perfect the adjustment under the eye.

ARTHUR W. BLACKLOCK, Newbridge,
Hardgate, Aberdeen.

SEA SICKNESS. SIR,-You lately gave a description of Mr. Bessemer's invention for 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, but are quite useless when carried out on 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. Bessemer'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), SIR,-I trust that Mr. Purkiss will pardon my alters with every ton of goods shipped or unshipped,

PARABOLISING SPECULA.

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Mechanical skill has 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 the result that the shocks experienced in running over hard roads occasioned continual breakages in the machinery. 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 and light itself, is climbs the severest gradients and draws enormous loads. It owes all its faculties and its exemption from the disabilities of other traction-engines to one The wheels, device, as simple as it is efficacious. which are of great width, are surrounded by tires of vulcanised india-rubber. These thick bar ds of indiarubber enable the road steamer to float over the surface of the ground without the slightest damage to the road, while they likewise protect the machinery tires between the wheel and the road aets, in fact, in from all concussion. The intervention of the elastic the same way as 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 finding none able to do work for which he required them, conceived and carried out the idea of providing the wheels of a steam-engine to run on common roads with india-rubber tires of immense thickness. When the first patent road 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 engines to send to all parts of the world, and the record of some of their performances, in the presence of engineers, agriculturists, and other practical men, will doubtless be found interesting. A three-horse power engine drew a boiler weighing 13 tons up an incline of 1 in 12, the ground 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, and again through a field covered to a depth of two feet

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