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ing the useful portion of the seed itself. This starch cells; but all of them ought to be present
latter is covered by a thin skin, penetrating into in a good sample. The chief use to which the mi-
its substance, and dividing it into irregular por-croscope can be applied, besides that of proving the
tions, called lobes. In carefully prepared cocoa, existence or otherwise of cocoa in the sample, as
entirely deprived of its husk, we have therefore above described, is the detection of starch which
only to look for the structures exhibited by the has been added as an adulteration. In Figs. 5
lobes, with their covering, and also those of the and 6 we show samples of cocoa adulterated with
embryo, which exists in one portion of seed, so common arrowroot, and also with sago flour. It
embedded as not to be separable in the process will be observed that the starch granules of these
of decortication. It thus follows that, under the substances are very much larger than those of
microscope, we may meet with three distinct cocoa, and at the same time exceedingly character-
structures in good cocoa, and these we will notice istic in shape. A short study of the drawings
seriatim
will render their forms familiar to our readers;
but we reserve any detailed description of them
for our article on the starches.

1. The Structure of the Thin Membrane.This is shown at a, in Fig. 7, and will be observed to consist of a mass of angular cells filled with oil, and much resembling the similar cells shown in the illustrations of genuine coffee. This membrane is usually of a brilliant dark golden colour, and the edges of the cells appear to stand out somewhat from the rest of the structure.

2. The Structure of the Lobes.-These consist entirely of ovate cells, filled with innumerable starch granules. The starch corpuscles are very small, and generally rounded, but no distinctive markings can be seen upon them, except by the highest powers of a very fine instrument, and even then, it is only on some of the granules that a spot or hilum can be observed. These cells of starch are also somewhat coloure, and are shown at b, Fig. 7.

The Structure of the Embryo.-This is seen at e, Fig. 7, and consists of broken and irregular tissues of cells, but which have a characteristic appearance. They are usually of a more delicate colour than the other structures, and frequently exhibit a most beautiful pink tint.

The most abundant of all these forms are the

To examine any sample of cocoa, it is only necessary to mount a few grains on a glass slid with water in the usual way, and look at it with a 4-inch power and A eye-piece. Masses of red colouring matter, such as ochre, &c., can be easily detected by the microscope; but for the detection of sugar, it is best to employ the process of solution already described. Half an ounce of good cocoa, stirred up in a pint of water, allowed to settle, collected on a piece of blotting paper, dried at a low heat, and weighed, should not lose more than 50 grains at the very most..

We cannot leave this subject without a word in favour of the excellent idea lately introduced by a few of the leading firms-namely, that of sell ing pure cocoa very finely ground and deprived to a considerable extent of its oil. As we have already pointed out, the richness of cocoa was a bar to its use by dyspeptic persons, and had thus lead to some authorities even approving of the "skilful chemical adjustment," or in plainer language, the "diluting and adulterating it with starch and sugar." But we most decidedly hold

that this admixture is not a chemical adjustment at all, as it is simply replacing one carbonaceous matter by another, and by so doing, diluting the whole substance, and thus materially reducing the percentage of the most important nitrogenous constituents of the cocoa. The most sensible way is undoubtedly to express a portion of the fat, and thus to leave an article in which all the remaining constituents are not only retained, but their percentage increased in a high degree. We would therefore counsel our readers to prefer, in every case, a cocoa thus prepared; as, if an increased proportion of fat is desirable, it can be easily attained by making their beverage with milk instead of water. These preparations have also the advantage of real cheapness, because a much smaller quantity may be used to make a cap of cocoa than with an ordinary starched and sugared article, especially if the substance be boiled for a minute or two to extract more thoroughly the soluble portions. We gave last month a comparison, on Dr. Johnston's authority, of cocoa and milk, which we now repeat with the addition of a column for cocoa prepared in this manner :

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collectors purchased the lower priced cocoas, a much worse picture would have been presented. The firm in question says, "You can hardly be aware of the enormous quantity of cocoa that is retailed at 6d. per lb., in square red packets; in many of these cocoas you would not find onefourth part of pure cocoa, the other three-fourths being composed of starch and sugar, though generally, molasses are substituted for the latter. We need hardly say that we do not manufacture this article, believing, as we do, that to sell such an article for cocoa, or even as prepared cocoa, is monstrous." So that we live and learn, and when we again advert to the subject of cocoa, we shall assuredly keep our eye on the 6d. red packets.

FRICTION IN STEAM CYLINDERS.
BY MR. P. JENSEN.

(Continued from page 274.)

friction per square inch of the same, is (say) 13lb. ing in the British Islands to a depth of 4000 feet
As the piston has two Ramsbottom steel rings, to be 37,300 millions of tons, this quantity of coal
and their total frictional area is 36 square inches, would supply the annual demand of 105 millions
we arrive at a friction co-efficient of 23 for con- for 355 years; and, taking the limit to deep mining
tinually but sparingly-greased steam. A piston to be a depth from the surface of 7000 feet, the
fitted with ordinary cast-iron packing rings further quantity of coal estimated to be workably
would, for this case, have it 24in. deep (instead to this depth was 57,222 millions of tons, which
of 24in. steel rings), and the friction co-efficient would extend the supply for a further period of
with same total friction would then become 535 years. The chief localities in the British
about 1-10th lb.
Islands where coal would probably be found at
greater depths than had hitherto been resc
were (1) the West Coast of Ayrshire, (2) the Wa
of Lancashire, (3) the East of Yorkshire, Der
shire Nottinghamshire and Staffordshire, and (4)
below the seams worked at present in the South
Wales basin.

The aim should, therefore, be to reduce the piston and the packing friction to a minimum. The slide valve friction is proportionately small, it having been shown that when working at full power it is only 1600ft. lbs., or about 1-39th of total friction. Nevertheless, if working with dry steam and without special lubrication, the slide valve friction may become considerable, even three times higher than stated. But worse than the friction itself are its results-namely, undue wear, scoring, &c., causing extra loss in fuel by leakage of fresh steam into the cylinder and the

WITII the view of placing the question of exhaust. The piston friction coefficient does in Belgium, which had attained the great depth of

friction in the steam cylinder in a clear light, it is proposed to work out an example from experiments made. With a 12-horse horizontal steam engine of good make-the cylinder 11in. by 16in. stroke-the indicator diagram gave 2-155lb. net average pressure for driving the engine alone. This, at 110 revolutions per minute, is equal to 1.88 horse-power. It is now proposed to calculate the friction of all the moving parts which can be conveniently got at, leaving piston and packing friction as the rest or

ba'ance.

Friction of Crosshead Pin.-2in. diameter, connecting rod mean pressure ns. me, say 2001b.,

crank
1

= 475, friction, co-efficient, 065;1 ×
4 X 4.75
105, which is a factor used several times; Mr.
McFarlane Gay's abbreviated formulæ used; then
with 110 revolutions-

200 x 65 x 1

Deep mining has been carried on much more extensively in Belgium than in England, there being only twelve pits of a greater depth than 1500ft. in the latter country, as compared with sixty-eight in the former. The deepest coal mine in the world was probably that of Simon Lambert, not under the same circumstances increase in 3489 feet. The deepest coal mine in England was the same ratio, at least, not in ordinary un- the Rosebridge Colliery, in Lancashire, which bas jacketed cylinders, in fact, were it not for the reached a depth of 2418ft., the temperature of much greater friction it absorbs per se, it would the coal at that depth being 93.5°. The distance not require extra lubricating material to the from the surface of the ground to the stratum of same extent as the slide valves, and for the invariable temperature might be taken at 60ft., and following reason:-While the steam passes the the constant temperature at that depth at 50%. The slide valve little or no liquefaction occurs, but accounts published between 1809 and 1810 of the opposite of this takes place in the cylinder several hundred experiments, relating to the temitself, as the work done by the steam causes a perature of coal and metalliferous mines, showed proportionate amount of liquefaction, and this the increase of temperature to vary from 1 for is the reason why many engines using rather every 45ft. to 1 for every 69ft.; the distance wet steam and not having steam jackets work from the surface at which the experiments without special means of lubrication. The were made varying from 100ft. to 1700ft. attraction of the walls of the cylinder tends to (Table III.) The results of more recent experithe deposition of a very thin film of water, which ments in England and on the Continent were acts as a lubricant. The case is, however, irregular, and showed an increase varying from different if the steam is dry, when little or no 1 for every 41ft. to 78ft., the distances from the condensation takes place, and it is then that surface being from 700 ft. to 2600 ft. (Table IV.) greasing the steam is of importance. If this is On comparing the experiments made at the two not done heavy wear and friction of the piston deepest English coal mines-viz. Rosebridge and will naturally result. It is better, for the sake of Dukinfield, it was found that the increase of temeconomy, to use steam just so much superheated perature due to the depth was much less rapid at × 1:05 = 03 × velocity, which that it retains its gaseous state during an expan- the latter colliery than at the former; and this sion carried as far as circumstances will admit. difference was assumed, in a paper ead recently The steam jacket will answer the same purpose. by Mr. Hull, to be due to an amount of heat being In connection with this point it may here be of lost at Dukinfield, owing to the heavy inclination of interest to mention an experiment, the particulars the strata, which was about 1 in 3, whilst at Roseof which the author has obtained. A compound bridge the coal seam was nearly level. (Table V.) high and low pressure engine, which drove the The relation of the position of the bottom of a machinery in the shop of Messrs. Humphreys, mine to the sea level influenced the temperature, was tried both with and without a steam jacket as shown in tables VI. and XXI. In the latter Friction of Crank Pin.-3in. diameter to satisfy the firm of the utility of the jacket. table the average increase of the temperature of 200 × 1625 X 065 The following are the results:-With stam in three mines of a high elevation was 18 for every the jackets it took 221b. pressure of steam in the 71-6ft., whilst the increase for three mines at some boiler to drive the engine at 135 revolutions per distance below the level of the sea was 1 for every minule; without steam in the jackets it took 62-3ft. 291b. pressure at the same speed; when the out- The experiments relating to the underground !des of the jackets were well felted 12 gallons of temperature of the air at the Rosebridge Colliery water were condensed in the jackets; without the showed an increase in the temperature of the air feiting 15 gallons were condensed in the same in passing from the downcast to the upcast shaft time. The 12 gallons, or the greater part of it, of from 55° to 63°; the air passing through workwent to supply heat to the steam in the cylinder, ings the temperature of which was 78°, and the while the 3 gallous apparently were lost by out- normal temperature of the coal being 93.5. The it-1 ward radiatios. experiments at Monkwearmouth (Table VIII.) showed the effect of a large volume of air in preventing a rise in temperature. At a distance of 1800 yards from the shaft, with 80,000 cubic ft. of air passing per minute, the temperature was 55°; whilst at a distance of 2600 yards from the shaft, with 10.000 cubic ft. of air circulating per minute, the temperature was found to be 67°.

12 × 8 × 4:75
6:28 × 110
4.75 X 12
Friction in Guides.

= 12 =

36ft. lbs. only. 0 X 07 X 7854

4.75

611ft. lbs.

10.2

X 277:

X 1·05 X ་ X 110

12

=

7·63 × S 33-3ft. lbs. Friction of Crank Shaft.-Here we must add the weight of crank, fly-wheel, &c. (say 90016.), 9-425

then

1100 X 15 X 065

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The waste heat of the uptake is often used for superheating purposes with or without steam jackets, but whether the one or the other is done, the saving of fuel is found to be very great. There is, however, one indispensable condition, namely, to keep the slide valve and piston from undue friction and wear, and the packings from getting hard and charred. Both these objects are attained in the most perfect manner by continuously greasing the steam before entering the ports. It is, therefore, highly necessary that all users of steam power should fulfil the condition by having recourse to an efficient steam lubricator, of which there are several now before the public.

(To be continued.)

COAL MINING IN DEEP WORKINGS.

The normal temperature of the coal might be estimated, from the results of experiments at Seaham Colliery, to exist in a main air channel, which had been exposed to the air for some time, at a distance of about 13ft. from the surface of the mineral. The highest temperature at which coal mines were worked was probably in Staffordshire and at the Monkwearmouth Colliery, where the temperatures varied from 80 to 85°. At the Clifford Tin Mine, in Cornwall, the temperature was 1208, in which the miners could only work for twenty-five minutes consecutively, this high temperature being due to the heat of the water issuing from the rock.

Adding these results together gives nearly 1000ft. lbs.; if we allow another 300ft. lbs. for the rest of the friction, including air resistance of fly-wheel, but not piston and stuffing-box friction, we see at a glance what an enormous proportion the latter bear to the whole. 2155 x 103-9 × 277 = 50848ft. lbs., out of which 49518ft. lbs. for piston and packings, and 1300lb. for all the rest. It may be said that this is not a fair way of putting the case, as the friction of the load will after this considerably when the engine is up to its power. Supposing, then, we say, as is actually the case, that this engine, when working full power, has a ten times higher average indicated pressure. The total number of foot pounds developed would be 508,480, out of which there are 13,000 for all the friction except piston and packings, the amount of the latter being as before 49,548. This, then, is a fair example, and here we find that out of the total friction of 49548+ 13000 = 62548, very nearly 4-5ths are absorbed Judging from the statistics of the past few years, by piston and packings. Here we are at the root the production of the British coal fie ds could not of the matter, and if we want to economise, here be considered to increase annually in a constantly In regard to the increase of temperature with is a wide field for every foot pound lost in fric-increasing ratio, as had been surmised, but might the distance from the surface, a careful comtion means a direct loss in money or fuel. It be estimated at an average "output "of 105 mil-parison of all the experiments quoted, and especi should be noticed that in this case the piston lions of tons yearly. Estimating the coal remain ally of those taken at a greater depth than 2000ft

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Of the three modes by which heat was lost by one substance and absorbed by another, viz., radiation, conduction, and convection, the only influence likely to come into action in a well ventilated mine of the depth stated would be that of convection. From the observations recorded, it would seem that, as a rule, when the temperature at the surface exceeded 66°, the temperature at the bottom of the pit was less than at the top; but when less than 66° at the top of the pit, an increased temperature was found at the bottom. The increase in the temperature, due to the increased density of the air in deep mines, was estimated at 1° for every 800ft., making the mean temperature of a pit 7000ft. deep about 598.

In regard to raising the coal, the probable limit from which it might be drawn at one lift was estimated to be about 900 yards, below which depth one winding engine at the surface, and one in the shaft would be required. An increase in the cost of sinking to great depths, and in the cost of producing the coal, must necessarily be expected; but since the selling price of coal would, to a great extent, be adjusted accordingly, this could scarcely be considered as a difliculty of much consequence.

led to the conclusion, that as far as could be judged | channel, as the length of the perimeter, and as a sufficient initial velocity to create in the air a from the experiments already made, the increase the square of the velocity of the air, and inversely solid resistance. His suggestion was that man of temperature would be 1° for every 55ft. in as the sectional area of the air-way. The action should be taught to fly as youths were taught to depth, from the stratum of invariable temperature. of these laws was demonstrated in the several swim in Germany-namely, by being suspended The data afforded by the experiments were so examples given, where it was shown that the by a rope, when he could in safety make experi irregular, that no law could be established as to power required to overcome the resistances varied ments with an artificial flying apparatus. He was the ratio of increased temperature augmenting or as the cube of the velocity. In drawing a com- quite willing to offer himself to the furtherance decreasing with increased distance from the sur-parison between furnace and mechanical ventila- of science as a rotatory pendulum bob, if the face, though the experiments at South Hetton and tion, it was calculated that, at a depth of about Aeronautic Society should think anything of his at Mouillelonge, as recorded in the paper, appeared 2,500ft., the two modes of ventilating were equal, suggestions. Mr. S. Harrison described some to indicate that the rise in temperature became while below this depth the furuace became the experiments which he had made with what he more rapid as the distance from the surface in- more effective power. called an aerial velocipede. His starting princreased. Assuming the rate of increase in temciple was that a screw-propeller set to act in the perature to be as previously estimated, the normal air merely churned the column of air upon which temperature of a mine 7000ft. deep would be it worked without making progress, but if it were 176. attached to a velocipede upon which a good rider could advance at the rate of ten miles an hour on a smooth road, the weight of the velocipede and its rider would be gradually diminished, and flight might thus at last be attained. His plan was to attach a plane of oiled silk to the veloci pede, and at the corners of this plane to fix his propellers. Mr. Harrison illustrated his theory The employment of machinery, in place of by chalk diagrams on the black board, and seemed manual labour, would probably be found very to convince the meeting that he had at least made beneficial in cutting and breaking down coal in some advance towards a true theory of aëroștadeep mines having a high temperature. Some tion.-The Chairman said that the last speaker of the coal cutting machines now at work were had really touched the true principle. If ever driven by compressel air, and the sudden de- they obtained perfect aërostation it must be by crease in temperature which compressed air means of an initial velocity acting on an inclined underwent on exhaustion had been thought aero plane. Mr. Harrison's idea seemed to lea likely to be of use in redncing the temperature up to both these desiderata, and he thought it of a mine. In reality, however, scarcely any would be worthy work for the society to test his reduction could be anticipated, since the theory by experiment. After some purely scienquantity of air exhausted bore so small a pro- tific discussion on the anemometer, a gentleman portion to an ordinary current of air, that the suggested that a light canoe working on the water effect on the temperature was only to be observed would be a better medium of experiment with the locally, and to a very slight degree. Of other propellers than the velocipede.-Mr. Louis Olrick, modes which had been proposed for facilitating consulting engineer, observed that the inventors the working of coal at great depths, neither that of the sewing machine had not succeeded by of casing the airways with non-conducting sub- endeavouring to imitate the old sewing process. stances, nor the employment of the electric light, They started on an entirely now principle, and nor the use of cold water and ice, could be aeronauts, if they wished to succeed, must do the anticipated to have any effect worthy of note. same. They must not follow or study the flight The hygrometrical experiments recorded showed of birds, but invent an entirely new plan of that the dryness of the air was considerably aërostation. What they wanted was patient increased with increased depth, especially in the experiment, and experiments cost money; aerostareturn air courses; and though this usually tion could therefore only be studied successfully caused a high temperature to be borne more by wealthy societies. The discussion terminated conveniently, it could not, in the case of the with the usual vote of thanks to the readers of the heavy labour required in working coal, be cal- papers. An instrument for measuring the pres culated to confer any benefi. sure of the air was exhibited, and explained by Dr. Smith, and very much commended by the Chairman.

The effect of the heat emitted by workmen, candles, explosion of gunpowder, &c., was estimated not to have any appreciable influence on the temperature of the air circulating in the mine. The experiments at Seaham showed the temperature of the return air to be 0.5° lower when the mine was in full operation than when the pit was off work, and when no lamps, workmen, &c., were in the workings. An unexplained cause of high temperature had been observed at several collieries, but more particularly at Monk wearmouth, where the temperature of the air on one occasion was found to be 95°, or upwards of 10° higher than the normal temperature of the mineral. The question, as to the effect of pressure upon deep workings, was unquestionably of great importance, and necessarily very speculative. The mode of working coal, suggested for a depth of 7000 ft., was arranged as far as possible in accordance with the principle, that the coal should be removed so as to present long lines of fracture, and should be so worked as to cause the superincumbent weight of the strata overlying the "goaf," or space where the coal was worked out, to have all its pressure upon such "goaf," and a minimum pressure upon the coal.

The increase in temperature in an underground air channel appeared from Table XXIII. to aver age about 15° for every 500 yards.

The question of ventilating a mine 7000ft. deep, to an extent sufficient to absorb the heat

Finally, it might be stated, that the question of coal, at greater depths than had hitherto been attained, could not be considered to be one which presented difficulties of any importance, nor was it one which required immediate consideration.

The author had endeavoured to prove that coal could be worked at a depth of 7000ft., but it would probably be centuries before such a sinking would actually be required, and improvements in the various

descriptions of mining machinery, especially such as were intented to facilitate the render mining to such a depth, as practicable as getting" of coal, would possibly before long the working of the deep mines of the present day. Commercially, as had been observed, the question would adjust itself to the requirements and the expenditure of the times.

THE AERONAUTICAL SOCIETY.

emitted by strata having a normal assumed tem-
perature of 176°, was one of the most important
in the inquiry, and the general results arrived at
might thus be enumerated: A. The temperature
of the air was estimated to increase from 59° at
the bottom of the downcast pit, to 65° at the
point where it reached the workings. B. The
length of time which would be occupied in cooling
the main air-way, to such an extent that the sides
of the road would have an average temperature of
622, and the normal temperature would be found
as far as 12ft. from the surface of the mineral. HE general meeting of members of the above
was calculated to be 40 days. C. The total
Teiner metting of members of the above
minute was found by calculation to be 45,320. chair was taken by Mr. Glaisher, of the Royal
number of units of heat emitted by the strata per theatre of the Society of Arts, Adelphi. The
D. The volume of air introduced at a tempera- Observatory. The Chairman, in opening the
ture of 650, and assumed to leave the workings ceedings, expressed his regret that the objects of
at a temperature of 89, necessary to carry away the society had not made much progress during
this number of units of heat, was calculated to the past year, bat hoped that the papers they
be 73,000 cubic feet per minute. E. Then, were about to have read would prove interesting.
the total quantity of air necessary for the ventil Mr. Walter Clure then proceeded to give his
tion of the pit to be 110,000 cubic feet per
minute, the power required to produce this
quantity would be 141 p.p., which represented
an average temperature in the upcast pit of 90,
for the attainment of which mean temperature,
temperature of 141 was required at the bottom
of the upcast pit. F. The quantity of fuel neces-
sary to raise the temperature of the return air
from 96 to 141°, was found to be 14:04 tons
every twenty-four hours.

a

The laws upon which the amount of power necessary to produce a certain quantity of air under every condition were stated to be as folJows:-The pressure per unit of sectional area of an air-way required to overcome the friction of the air, varied directly as the length of the air

pro

views on aeronautic science. He contended, as
we understood, that manual flight would be the
ultimate triumph of aeronauts, and reminded the
meeting that the ancients had devoted their atten-
tion exclusively to that branch of the subject. He
combated the many popular errors that prevailed
as to the indispensable conditions of manual
flight, contending that the means by which birds
were enabled to fly were as yet very imperfectly
known, even to the scientific. He believed that

A THEORY OF NEBULE AND COMETS.*
(Concluded from page 273.)

theory gives of comets.
WILL now examine what account the present

There are some reasons for supposing that the

snu itself is a nebulous star, or that it is enveloped of the zodiacal light, the bands of meteoric by matter extending to an immense distance beyond its visible photosphere. The phenomenon matter passed through by the earth in its revolution about the sun, the retardation experienced by comets, all point to this conclusion.

We cannot suppose that this envelope consists of solid or liquid matter only, without the presence of gaseous matter; for at no known temperature can liquid or solid matter exist in a vacuum without evaporation. If, then, we suppoen for witment that solid mattor hell, we supitself. This atmosphere will be very rare and out being enveloped by gas, it will immediately begin to evaporate and form an atmosphere about very extensive, as the central mass, being comparatively, very small, will exert but a feeble

attraction on it.

Or, again, if we suppose that meteoric matter phere by evaporation, it would do so in another

unenveloped by gas would not acquire an atmos manner; for it is certain that some of the meteoric bands approach very near to the sun in their perihelion. These would attract to themselves a part of the sun's atmosphere; which they would carry away with them on their departure; and portions of this they would, in their turn, part with to every meteor which came within a sufficiently small distance from them.

Graham has found that meteoric matter which

has fallen to the earth, gives evidence of having been exposed, when at a high temperature, to hydrogen existing under a pressure of severa!

man would ultimately be made a flying animal,
but there were certain artificial difficulties to be
overcome before the much-desired object was
attained. Air was, as a matter of fact, as solid atmospheres.
as any other material, and it only required that
the person seeking to fly should be able to get up Leeds School, in the Philosophie & More.
By A. S. DAVIS, BA., Mathematical Master.

We conclude, then, that the sun is surrounded and the light shading denote, respectively, the by an envelope of gas, which is not a true solar large mass of rarer gas and the small mass of atmosphere, but is the aggregation of the atmospheres of numberless meteoric bodies revolving around it.

Now M. Hoek has shown that comets are detached portions of large masses of matter; and it has been suggested that these large masses may be nebulæ. Admitting this, a comet, before its entrance into the solar system, will consists of a solid or liquid nucleus surrounded by a large mass of very rare invisible gas. On its approach to the sun, the nuclens will make its way most easily into the solar envelope, and the comet will enter with its tail directed away from the sun. A chemical combination will take place between the tail of the comet and some of the gaseous elements of the solar envelope; and where this combination occurs, the gases will become visible from the light evolved, and, if the compound formed be in a solid or liquid state, from the light also which it reflects from the sun-or if, as probably would be the case, the matter be in a state of minute subdivision, from light scattered by that kind of dispersion which Professor Tyndall has lately shown is produced by finely-divided matter.

On passing through its perihelion, the comet loses a great part of its tail, which soon cools down and becomes mingled with the rest of the solar envelope.

On leaving the sun, the tail begins to increase, from the addition to it of matter rendered gaseous by the heat of the sun. Those parts of the gas where chemical action has taken place being heated, and therefore rendered specifically lighter than the unheated invisible gases, will have a tendency to escape out of the solar envelope in addition to that which they possess from their momentum in common with the rest of the comet. Hence the comet will depart with its tail directed away from the sun.

The hollow appearance of many comets, and isolation of the nucleus from the vertex of the coma, are in accordance with this theory.

On a comet's approach to the sun, it often happens that a tail of immense length is formed in a very few days. It is usually supposed that the matter forming the tail has all been projected from the head within the time of its first becoming visible, and consequently that it has moved with enormous velocity in a direction opposed to the sun's attraction. Hence it has been conjectured that the matter forming the tail is not subject to the same mechanical laws as those which govern all other known matter.

On the present hypothesis there is no need to assume this enormous velocity. A comet, in fact, enters the solar envelope with a tail of invisible gas. It may be that chemical union cannot take place between this gas and the sun's envelope until the heat of the sun, acting on the head of the tail, has set up chemical action or combustion -until, in short, the comet has been lit by the sun's heat. When once combustion has commenced, it would spread into the tail with prodigions velocity.

Tails of comets have been observed to form with enormous speed only on their approach towards the sun. The tails which form when a comet is receding from the sun are produced with comparative slowness: this we should expect ; for in this case there is not already in existence a tail needing simply to be lit to become visible.

The more any portion of the gas of a comet becomes removed from the nucleus, the greater will be the volume it occupies, because of the diminution in the attractive force of the nucleus. This will account for the spreading shape of the tail of a comet.

We may explain in a similar way the increase in the size of a comet as it recedes from the sun; for the pressure of the solar envelope upon it will become less as its distance from the sun becomes greater. M. Valz has attempted an explanation of this fact in a somewhat similar manner. He conceives that the increase in the size is due to a diminution in the pressure of the ætherial medium, which he supposes to be denser in the neighbourhood of the sun than elsewhere. Sir J. Herschel objects to this explanation, on the ground that we must suppose that the ether does not pervade the matter composing the comet.

This objection does not amply to the present explanation.

The following figure is intended to represent a section of the nebula (Lassell, pl. 2. Fig. 9, Royal Astronomical Society's Memoirs, vol. xxxvi.) shown on page 273.

denser gas. The dark shading denotes the part rendered visible by chemical action.

Immediately below is shown the mode of formation of an annular nebula. The mass of gas

indicated by the medium shading must be smaller than that indicated by the light shading, and also rarer. If it were denser, it would ultimately enter into it, and a spiral nebula would be formed. The dark shading, as before, shows the visible part. This represents the formation of a spiral nebula.

Two masses approaching each other move is parabolas, having their centre of gravity as common focus. On approaching each other they are drawn out into an elongated form; and if a collision occurs, the common boundary will be shaped as in the figure.

SCIENCE FOR THE YOUNG.

BY THE REV. E. KERNAN, CLONGOWES COLLEG (Continued from page 269.) CHAPTER III.

VARIOUS TERMS, EXPLANATION OF. HERE cannot be a greater obstacle to progress

stand the terminology (names, words, expressions) it requires Mechanics as every branch of science has its technical terms. Besides, it frequently calls in the aid of mathematical language and argument. With these latter, the student is supposed to be sufficiently familiar; there re mains, therefore, nothing to be explained, but the mechanical terms. Some of the more obvious of these have already, of necessity, been used, Yet a few words of definition may not be amiss. and might be considered as sufficiently understood. Matter.-Every thing known by the senses. Body.-Every quantity of matter. Atoms.-The mechanically indivisible parts of

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resulting is caused by the force communicated to the cue. After a while, the ball stops; the table exerts a force which gradually modifies the velocity of the original motion, and finally stops the ball. Let it meet a block of marble A, Fig. 47, it changes its course; the marble, without

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Lassell gives several drawings of S shaped tion of the original motion. Force, therefore, nebula having nuclei in the middle.

does not necessarily imply an evident motion; in the effort at motion lies the exertion of the force.

Force, Elements of.-They are chiefly three; direction, the line along which the force acts, or tends to make a body move; intensity, the amount of force exerted; point of application, that part of the body at which the force is exerted. All three elements are represented in mechanics by lines. In one symbol, a line, the three great questions regarding a force are answered. For a line is eminently fitted to represent the direc tion and point of application; and the relative intensities of two or more forces can be shown by the proportional lengths of line. Thus, suppose two forces acting upon a body, at fixed points and at a given angle: one force three times as strong as the other. Lines will show these conditions to the eye. Let A B and CD, Fig. 48,

The above diagram is intended to represent the be the two forces acting on the body M; and A

The parts represented by the medium shading mode of formation of a nebula of this kind.

B = 3 x C D. Now A B is made to show the

- direction and point of application; and repre-
sents by its length the relative intensity of the
greater force.
CD does the same for the lesser
Besides this concise clearness, lines are

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force.

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subject to the control of mathematical principles. What is mathematically true of them as lines, can be applied to the forces which they represent, and thus exact laws can be established on the unerring basis of mathematical argument. In experimental proofs of the laws of forces they are usually represented by weights, which is very convenient -weights can be so well compared and are so

familiar.

stood by mechanics. It is much longer than the that, in this section, the object always is, to de
other two, as therein are established the prin- termine how equilibrium may be produced, or as
ciples of the science in its widest sense. The it is usually expressed the "conditions of equi-
second and third chapters show how far these librium" of forces applied to a body.
principles apply to the liquid and gaseous states,
and how they are modified by the nature of
those states. Each chapter shall be divided into
four sections; the first, the study of the general
properties of matter, with special regard to the
state under consideration; second, bodies at
rest; third, obstacles to theoretic truth; fourth,
bodies in motion from the action of forces.

CHAPTER I.

MECHANICS OF SOLIDS.

to

In this chapter the great important subjects are the laws of bodies at rest, or "Statics," and the laws of bodies under the action of forces, or "Dynamics." The section on "obstacles " theory is of absolute necessity, as showing how far the theoretic principles shall be modified in practice. With regard to the solid state, as concerns the general properties of matter, little remains to be added in an elementary course, to what has been already said. With more advanced students this section would hold a very important position as being the exact place for the study of "molecular action," the principles of which should be constantly used throughout the entire course. Such study for beginners is quite impossible. However, as this section is of immense importance in the two other states, for uniformity the division as indicated above, is even Nintroduced here.

Requirements.-The requirement of a force is its relation to all positions in space. In the plainest terms, this signifies what a force wants to do with a given body in regard to some fixed position, or to all positions in space. Example: the force Р acting upon the

body A, Fig. 47, is represented in

FIG. 49

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and

SECTION I.

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Q represent two forces parallel acting upon the body A at the pivots a a. Move the rods to an angular position, the dotted lines; their direction prolonged must meet somewhere in a point B. Their new action at a a is the same as if they were transferred to the point B. is an admitted principle that if a force applied to a body be transferred to any point of space in the line of its direction, its action is not changed, provided that point of space be rigidly joined to the body. There is a simple experimental proof of the principle. Suppose a body A, Fig. 53, under the action of three forces P Q R in equilibrium. The forces P and Q hang parallel from the corners; R is a weight at the back (seen below) which takes effect over the pulley p. The forces are now changed to an angular position, by drawing the cords over the side pulleys

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direction and intensity by the PA; its requirement regarding the position a is indicated by the perpendicular Pp on a from P, the point of complete exhaustion of the force. Therefore, the requirement in this example answers to the question: what does the force P want to do with the body A, as regards the form, retains this form. On the other side, many forces P and Q is not changed by the transfer. It wants to place the body at a solid, or rather all that deserve to be called solids. It is clear, moreover, that the point B might be

MATTER, GENERAL PROPERTIES. Solid bodies may be said to possess all the general properties of matter in a greater or less degree. There are two however, which are very a a, and the weight R adjusted to suit the new marked in the solid, when contrasted with the conditions of equilibrium, which was disturbed liquid and gaseous state. And these are-figure-why ?-no matter, just for the moment. Now, and gravitation. Some solids, as stated, affect whether the forces P and Q be kept pinned to certain definite forms; but all are indifferent to the corners, or allowed to act along the prolongaany form which external causes may give them. tion of their direction at B, the state of equilibrium remains. Therefore the action of the The most complicated crystal, cut into a simple

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are quite unaffected by the form of a recipient; nor can they be made to take its form except by great force. A mass of iron, has the same form when cold, no matter what the shape of the reof Paris, has proved it by many and long-concipient which contains it. Still, it is known, Tresca produce, or could pro-made to flow, somewhat like liquids. tinued experiments, that metals (cold) can be duce if allowed to act. In Fig. 50, let P and Q be two forces acting on the body A-(forces will always be represented by a single letter at the "point" of their lines.) It shall be shown later, that when the forces are allowed to act, the body A will

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Composition. The replacing two or more forces by one resultant. P and Q replaced by R. Decomposition.-The dividing up of one force into two or more, that will produce the same effect as the original one. R divided into P and Q. System of Forces.-A number acting at one time together upon a body.

Equilibrium.-There is equilibrium when a body under the action of a system of forces is at rest-so-called rest; for there is no such thing as absolute rest. That, in its strict sense, would suppose an absence of all force, which is not possible in the ordinary state of bodies. Absolute rest has another meaning, as opposed to relative rest, of which there is no question now. Mobility. The power of moving which some bodies have, or the allowing themselves to be moved. Immobility is the opposite. Motion. The exertion of the power of moving, or the being forced to change position in space.

PART II.

MECHANICS OF THE THREE STATES. THE preliminaries being well understood, there will be nothing to divert the attention from working out the definition of mechanics, the study, &c. This part will contain three chapters, one for each of the states. The first of the three, of "solids," treats what is usually under

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anywhere outside the limits of the body A, pro-
vided the condition of rigid connection, between
the point B and the
body A be secured.
To the body A, Fig.
54, two forces are ap-
plied at a a. These
forces are transferred
till they meet, and
the point of joining
rigidly connected by
the bar C to A. There
is no change of equi-
librium. All this

more upon the pan of a principles of Statics ("the conditions of eqni-
balance, when the rod
is (screwed) perpen- the two titles "forces applied to a point;"'.
librium") are therefore to be classed under
dicular a, than when
"forces applied parallel to a rod."
it lies horizontal b.
The reason of remark-
ing these two well-
known facts will ap-
pear in the Mechanics
of liquids.

Other properties might be mentioned, of which the effects in the solid, contrast with those in the liquid and gaseous state; but the two just spoken of are sufficient to direct the mind to other facts, of themselves quite obvious.

SECTION II.
STATICS.

To treat this matter as it should be, the various principles of equilibrium would be contained in a series of propositions, like those of mathematical works, to be proved slowly, step by step. This, the true method, is to be found abundantly in a variety of excellent works on Statics. But, besides other considerations, the students for whom this course has been compiled, are not supposed to have time at their disposal sufficient for protracted study. No more, therefore, than the great principles can be discussed; to these, however, may be reduced, at least in a general way, all the practical applications which have given to mechanical study its paramount importance.

It will be well to impress deeply on the mind,

Into a course of Statics is always introduced the explanation of the so-called, "machines," memuch used for motion, and are often exhibited chanical powers. And though, in practice, they are in motion, they are not out of place in statics. For they are constructed according to, and act by, the principles of equilibrium. Besides, they can be viewed only as a means of obtaining equilibrium. That an increase of force produces motion for practical purposes does not make any change in the conditions of equilibrium of the

machine.

(To be continued.)

GALVANIC BATTERIES.*
HERE have been within the past few years a

some of them have been used to a very considera-
ble extent, among which may be mentioned the
Sulphate of Mercury and Graphite Battery, by
Marie Davy; the Peroxide of Magnanese, by
Leclanche, and the Gravity Battery, by Callaud.
These elements are referred to in Ganot's physica
as "new batteries." At one period the Telegraph
Department used either the sulphate of mercury
or the peroxide of manganese element, and, later,
the Callaud was introduced into the service :
each of these inventions had its advocates. Every

* From the "Electric Telegraph Review."

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