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ing the useful portion of the seed itself. This latter is covered by a thin skin, penetrating into its substance, and dividing it into irregular portions, called lobes. In carefully prepared cocoa, entirely deprived of its husk, we have therefore only to look for the structures exhibited by the lobes, with their covering, and also those of the embryo, which exists in one portion of seed, so embedded as not to be separable in the process of decortication. It thus follows that, under the microscope, we may meet » it.h three distinct structures in good cocoa, and taste we will notice teriatim

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 «mall, and generally rounded, but no distinctive markings can be seen upon them, except by the highest powers of a very fine instrument, andevon 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 ¿, Fig. 7.

The Structure of the Embryo.—This is seen at t', 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

starch cells ; but all of them ought to be present in a good sample. The chief use to which the microscope can be applied, besides that of proving the existence or otherwise of cocoa in the sample, as above described, is the detection of starch which has been added as an adulteration. In Figs. 5 and 6 we show samples of cocoa adulterated with common arrowroot, and also with sago flour. It will be observed that the starch granules of these substances are very much larger than those of cocoa, and at the same time exceedingly characteristic in shape. A short study of the drawings will render their forms familiar to our readers; but we reserve any detailed description of them for our article on the starches.

To examine any sample of cocoa, it is only necessary to mount a few grains on to glass slidwith water in the usual way, and look at it with a pinch 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 uot loso 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 selling pure cocoa very finely ground and deprived to B 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 aad 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 carbonaceoae matter by another, and by so doing, diluting the whole substance, and thus materially reducing the percentage of the most important uitrogt».. 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 » 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 ihe lower priced cocoas, a much worse picture wonld have been presented. The firm in question says, "Yon can hardly he aware of the enormous quantity of cocoa that is retailed at6d. per lb., in square red packets; in many of these cocoas you would not find onefourth part of pnre 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 Od. red packets.

FRICTION' IN STEAM CYLINDERS.

By Mr. P. Jensen.

(Conlinved from page 274J

WITH the view of placing the question of diction in the steam cylinder in a clear light, it is proposed to work ont an example from experiments made. With a 12-horae horizontal steam engine of good make—the cylinder 11 Jin. by IGin. stroke—the indicator diagram gave 2' 1551b. 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, ]e.wing piston and poking friction as the rest or balance.

Friction of Crouhead Pill.—2in. diameter,

connecting roil

mean pressure ■ n s: rue, say 2001b..—

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friction per square inch of the *ame, is (say) 1 Jib. As the piston hns two Ramsbottom steel rings, and their total frictional area is 36 square inches, we arrive at a friction co-efficient of '23 for continually but sparingly-greased steam. A piston fitted with ordinary cast-iron packing rings would, for this case, have it 2Jin. deep (^instead of 2£in. steel rings), and the friction co-efficient with same total friction would thon become about 1-10th lb.

The aim should, therefore, be to reduco 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 1 (100ft. lbs., or about l-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 itsolf are its results—namely, undue wear, scoring, &c, causing extra loss in fuel by leakage of fresh steam into the cylinder and the exhaust. The piston friction coefficient docs not under the same circumstances increase in the samo ratio, at least, not in ordinary unjacketed cylinders, in fact, were it not for the much greater friction it absorbs per te, U wonld not require extra lubricating material to the same extent as the slide valves, and for the following reason :—While the steam passes the slido valve little or no liquefaction occurs, but the opposite of this takes place in the cylinder itself, as the work done by the steam causes a proportionate amount of liquefaction, and this is the reason why many engines using rather wet steam and not having steam jackets work without special means of lubrication. The attraction of the walls of the cylinder tends to the deposition of a very thin film of water, which acts as a lubricant. The case is, however, different if the steam is dry, when little or no condensation takes place, and it is then that greasing the steam is of importance. If this is not done heavy wear and Iriction of the piston will naturally rtsult. It is better, for the sake of economy, to use steam just so much superheated that it retains its gaseous state during an expansion carried as far as circumstances will admit. The steam jai ket will answer the same purpose

In connection with this point it may here be of interest to mention an experiment, the particulars of which the author has obtained. A compound high and low pressure engine, which drove the machinery in the shop of Messrs. Humphreys, was tried both with and without a steam jacket to satisfy the firm of the utility of tho jacket. The following are the results:—With at am in the jackets it took 221b. pressure of steam iu the boiler to drive tl,e engine at 135 revolutions per minule; without steam in the jackets it took 291b. pressure at the same speed; when the outt the jackets were well felted 12 gallons of

Frivlio't.—Tho acting areu exposed to pressure being 18 .square inches, and friction

15 co-efficient l-10ih, (hen, 1\S x— X 110 X 2155 =

12 lliOft. lbs.

Adding these results together gives nearly 1000ft. lbs. j if we alio* another 300ft. lbs. for the rest of the friction, including air resistance of dy-wheel, but not pUton and stuffing-box friction, we see at a glance what an enormous oroportion the latter bear to the whole. 2-165 X 103-9 x 277 = 50848ft. lbs.,outof which 49518ft. lbs. for piston and packings, and 13001b. for all the res1. It may be said that this is not a fair way of putting the ca-e, as the friction of the load will alter this considerably when the engine w up to its power. Supposing, then, we say, as is actually the case, that this engine, when working full power, bus 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 aniountof the latter being as before 19,548. This, then, is a fair example, and here we find that out of tho total friction of 49548 -+13000 = 02548, very nearly 4-5ihs are absorbed by piston and packings. Hero we ure at the root of the matter, and if we want to economise, here i.i a wide field for every foot pound lost in fric•ion moans a direct loss iu money or fuel. It "houUl be noticed thut iu this case the piston

ing in the British Islands to a depth of 4000 feet to be 37,300 millions of tonB, this quantity of cool would supply the annual demand of 105 millions for 355 years ;and, taking the limit to deep mrnioi to be a depth from the surface of 7000 feet, the further quantity of coal estimated to be workabh to this depth was 57,222 millions of tons, whiri would extend the supply for a further period •■-! 535 years. The chief localities in the Brit -i Islands where cool would probably be found at greater depths than had hitherto been rearhsi were (1) the West Coast of Ayrshire, (2) the W« of Lancashire, (3) the East of Yorkshire, Derbyshire Nottinghamshire and Staffordshire, and (\) below the seams worked at present in the Sooth Wales basin.

Deep mining has been carried on much more extensively in Belgium than in England, them 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, in Belgium, which had attained the great depth of 3189 feet. The deepest coal mine in England was the Rosebridge Colliery, in Lancashire, which has reached a depth of 2418ft., the temperature of the coal at that depth being 935°. The distance from the surface of the ground to the stratum of invariable temperature mightbe taken at 60ft.,and the constant temperature at that]depth at 500.'-, The accounts published between 1809 and 1840 of several hundred experiments, relating to the temperature of coal and metalliferous mines, showed tho increase of temperature to vary from 1" for every 45ft. to 1" for every 69ft. ; the distant* from the surface at which the experiments were made varying from 100ft. to ]70OfL (Table III.) The results of more recent experiments in England and on the Continent were irregular, and showed an increase varying from 1" for every 41ft. to 78ft., the distances from the surface being from 700 ft. to 2600 ft. (liable IV.) On comparing the experiments made at the two deepest English coal mines—viz. Rosebridge and Dukiutield, it was found that the increase of temperature due to the depth was much less rapid at the latter colliery than at the former ; and this difference was assumed, in a pa;>er lead recently by Mr. Hull, to be due to an amount ot heat being !<^t at Dukinfield, owingtothebeavvincHnaUonof the strata, which was about 1 in 3, whilst at Rosebridge the coal seam was nearly level. (Table V.) The relation of the position of the bottom of a mine to the sea level influenced the temperature, as shown in tables VI. and XXI. In tho latter table the average increase of the lempcralure of three minesof a high elevation was 1" for every 71'61't., whilst tho increase for three minesat some distance below the level of the sea was 1 c for every 62 :tft.

The experiments relating to the underground temperature of the air at tho Rosebridge Colliery

wavr were condensed in the jackets ; without tho j showed nn increase in the temperature of the air felling 15 gallons were condensed in the same in pa-sing 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, I ings the temperature of which was 78Q, and the while the 3 gallons apparently were lost by out- | normal temperature of the coal being 93.5". The ward radiation-. , experiments at Monkwearraouth (Table VIII.)

The waste heat of the uptake is often used for showed the effect of a large volume of air in pre

superheating purposes with or without steam jackets, but whether the one or the other is done, the saving of fuel is found to bo 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 oharred. Both thesa objects ore attained iu 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 tho 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.

WE now give, as promised last week, an abstract of a second paper on " Coal Mining," wiiich was recently read before the Institution of Civil Engineers, by Mr. Emerson Bainbridge, Stud. Inst. C.E.

In this communication the piincipal conclusions arrived at were to the following oileet:—

Judging from the statistics of the pastfewyears, the production of the British coal tie ds could not be considered to increase annually iu a constantly increasing ratio, as had been surmised, but might

bo uMimuit'd at an avenge "output " ui 105 mil-
lions of tons yearly. Estimating the coal remain- ally of those taken at agreater depth than

venting a rise in temperature. At a distance of 1800 yards from the shaft, with 80,000 cubic ft. of air passing per minute, the tempemtnre 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°.

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 ot which coal mines were worked was probably in Staffordshire and at the Monkwearmouth Colliery, where the icmperntnres varied from 80u to 85°. At the Clifford Tin Mine, in Cornwall, the temperature was 120a, in which the minors could only work for twenty-five minutes consecutively, this high temperature being due to the heat of the water issuing from the rock.

It would appear, from the contradictor^ results of the experiments relating to the temperature of different minerals, that no rulo could be-laid down.

It was probable, however, that the temperature of mines was effected to some extent by the varying conductive power of different miuerals.

In regard to the increase of temperature with the distance from the surface, a careful comparison of all tho experiments quoted, »ud espeei

000ft led to the conclusion, that as furas could be judged from the experimente already mide, the increase of temperature would be Io for every 50ft. in depth, from the stratnm of invariable temperature. The date, aff Td >d by the experiments were so irregubr, that no law could be established as to the rati > of increased temperature augmenting or decreasing with increased distance from the surface, though the experiments at South Fletton and aiMouillelonge, as reenrdedin the paper, appeared to indicate that the rise in temperature became more rapid as the distance from tha surface increased. Assuming the rate of increase in temperature tn be as previously estimated, the normal temperature of в mine 7000ft. deep would bo

írtw.

Of the three mides by which heat was lost by one substance and absorbed by another, viz., radiation, conduction, and convection, the only influence likely to come intoiaction 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 6t>°. the temperature at the bottom of the pit was less than at the top ; but when less than 6*5" 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 everv 8O0ft., making the moan temperature of a pit 7000ft. deep about 59".

The effect of the heat emitted by workmen, candles, explosion of gunpowder, &c, was estimated n>>t 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 tobe 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 working«. An unexplained cause of high temperature had been observed at scveralcollierie-, lint more particularly at Monkwearmouth, where the temperature of the air on one occasion was found to be g.")", or upwards of 10° higher than the normal temperature of the mineral. The question, as to the effect of pressure upon deep workings, wns unquestionably of great importance, and nece-sarily 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 shonld.be removed so ns to present long lines of fracture, and should be so worked as to сап-e the superincumbent weight of the strata overlying the " goaf," or space where the coal was worked out, to have all its pressure upon sneb "goaf," and a minimum pressure upon the coal.

The increase in temperature in an underground air channel appeared from Table XXIII. to average about 1-5° for every 500 yards.

The qnestion of ventilating a mine 7010ft. deep, to an extent sufficient to absorb the heat emitted by Btrata having anormal assumed temperature of 17tj°, was one of the most important in the inquiry, and the general results arrived at miL'ht thns be enumerated : A. The temperature of the air was estimated to increase from 59° at the bottom of the downcast pit, to 05° 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 62"", and the normal temperature would be found as fares 12ft. from the surface of the mineral, was calculated to be 40 days. C. The total nnmber of units of heat emitted by the strata per minute was found by calculation to be 45,320. D. Tiie volumo of air introduced at a temperature of G5°, and assumed to leave the workings at a temperature of 89", necessary to carry away this number of units of heat, was calculated to be 73,000 cubic feet per minute. E. Then, the total quantity of air necessary for the ventil ition 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, a temporatare of 141° was required at the bottom of the upcast pi'r. F. Tho quantity of fuel necessary to rais« the temperature of the return air from 9(1° to 141°, was found to be 1401 tons every twenty-four hours.

The laws upon which the amount of power necessary to produce a certain quantity of air under every condition wero stated t > be as follows —The pressure per unit of sectional area of un air-way required to overcome the friction of the air, varied directly as the length of tho air

channel, as the length of the perimeter, and as the square of tho velocity of the air, and inversely as the sectional area of the air-way. The action of these laws was demonstrated in tho several examples given, whore it waa shown that the power required to overcome the resistances varied as the cube of the velocity. In drawing a comparison between furnace and mechanical ventilation, it was calculated that, at a depth of about 2,500ft., the two modes of ventilating were equal, while below this depth tho furuace became the more effective power.

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

The employment of machinery, in place of manual labour, would probably be found very beneficial in cutting and breaking down coal in deep mines having a high temperature. Some of the coal cutting machines now at work were driven by compresse 1 air, and the en hi ;n decrease in temperature which compressed air underwent on exhaustion had been thought likely to be of use in reducing the temperature of a mine. In reality, however, scarcely any reduction ooull be anticipated, since the quantity of air exhausted bore so small a proportion to an ordinary current of air, that tho effect on the temperature was only to be observed locally, ami to a very slight degree. Of other modes which had been proposed for facilitating the working of coal at great depths, neither that of casing the airways with non-conducting subit mees, nor the employment of the electric light, nor the nse of cold water and ice, could bo anticipated to havo any effect worthy of note. Tho hygrometrioal experiments recorded showed that the dryness of the air was considerably increased with increased depth, especially in the return air courses; and though this usually caused a high temperature to be borne more conveniently, it could not, in the c;isc of the heavy labour required in working coal, be calculated to confer any benefi'.

Finally, it might be stated, that the question of coal, at greater dopths than had hitherto been attained, could not be considered to bo 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 "getting "of coal, would possibly before long render mining to such a depth, as practicable as the working of the deep mines of the present day. Commercially, as had been observed, the question would adjust itself to the requirements anil the expenditure of the times.

a suffioient initial velocity to rreate in the air a solid resistance. His snggestiun was that man should be taught to fly as youths were taught to swim in Germany—namely, by heilig suspended by a rope, when he could in safety m ike experiments with an artificial flying apparatus. He wis quite willing to offer himself to the furtherance of science as a rotatory pendulum bob, if the Aeronautic Society should think anything of his suggestions.—Mr. S. Harrison described some experiments which he had made with what he called an aerial velocipede. His starting principle was that a screw-propeller set to act in the air merely churned the column of airnpon which it worked without making progrès hnt if it wre attached to a velocipede npon which a good rider could advance at the r.tte 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 bo attained. His plan was to attach a plane of oiled silk to the velocipede, and at the corners of this plane to fix his propellers. Mr. Harrison illustrated his theory by chalk diagrams on the black board, and seemed to convince the meeting that he had at least made some advance towards a true theory of aerostation.—The Chairman said that the last speaker had really touched the true principle. If ever they obtained perfect aerostation it mnst be by means of an initial velocity acting on an inclined aero plane. Mr. Harrison's idea seemed to lea up to both these desiderata, and he thonght it would be worthy work for the society to test his theory by experiment. After some purely scientific discussion on the anemometer, a gentleman snggested that a lijrht canoe working on the water would be a better medinm of experiment with the propellers than the velocipede.—Mr. Louis Olrick, consnlting engineer, observed that the inventors of the sewing machino had not succeeded by endeavouring to imitate tho old sewing process. They started on an entirely new principle, and aeronauts, if they wished to succeed, must do the same. They must not follow or study the flight of birds, but invent an entirely new plan of aerostation. What they wanted wns patient experiment, and experiments cost money ; aerostation could therefore only be studied successfully by wealthy societies. The discussion terminated with the usual vote of thanks to the readers of the papers. An instrument for measnrlng the pressure of the air was exhibited, and explained by Dr. Smith, and very much commended by the Chairman.

THE AERONAUTICAL SOCIETY.

THE general meeting of members of the above society was held on the 3rd inst., in the theatre of the Society of Arts, Adelphi. The chair was taken by Mr. Glaishcr, of the Royal Observatory. The C.mirman, in opening the proceedings, expressed his regret that the objects of the society had not made much progress daring the past year, but hoped that the papers they were about to have read would prove interesting. Mr. Walter Clure then proceeded to give his views on aeronautic science. He contended, as wo understood, that manual flight would bo the ultimate triumph of aeronauts, and reminded the meeting that the ancients had devoted their attention 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 tho scientific. He believed that 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 as any other material, and it only required that the person seekiug to fly should he able to get up

A THEORY OF NEBUIJE AHTJ COMETS." (Concluded from, page 273.)

I WILL now examine what amount the present theory gives of comets.

There are some reasons for snpposing that the snu itself is a nebulous star, or that it is enveloped by matter extending toan immense distance beyond its visible photosphere. The phenomenon of the zodiacal liiht, the bands of m«teor'umatter passed through by the earth in its revolution about the sun, the retardation experienced by cornels, all point to this conclusion.

We cannnt suppose that this envelope consists of solid or liquid matter only, without the presence of gaseous matter ; for at no known temperature can liqnkl or solid matter exist in n vacuum withont eviporatirm. If, then, we suppose for a moment that eolid matter exists without being envelope i by ga«, it will immediately begin to evaporate and form an atmosphere about itself. Thin atmosphere will be very rare aDd 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 unenvcloped by gas would not aoqnire an atmosphere by evaporation, it would do so in another manner ; for it is certain that some of tho meteoric bands approach very near to the sun in their perihelion. These would attract to themselves а part of the sun's atmosphere; which they would carry away with them on their departure; and portions of this they weuld, in their tnrn, 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 pre««nro of severa! atmospheres.

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We conclude, then, that the sun is surrounded by an envelope of gas, which is not a true solar atnospberc, 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 nebulae. 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 nucleus 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 matfor 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 two 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 prodigious 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 astherial 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 (Lasaell, pi. 2. Fig. O.Ruynl Astronomical Society's Memoirs, vol. xxxvi.) shown on page 273.

The parts represented by the medium shading

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Two masses approaching each other more ti parabolas, having their centre of gravity as i common focus. On approaching each other the. are drawn out into an elongated form; and if J collision occurs, the common boundary will k shaped as in the figure.

SCIENCE FOR THE YOUNG.

Sytherev. E. Kernan, Clongowes Collet.:

(Continued from page 269.)

CHAPTER III.

Various Terms, Explanation Of.

THERE cannot be a greater obstacle to progress in any course of study, than not to understand the terminology (names, words, expressions) it requires Mechanics as every branch ofsciena has its technical terms. Besides, it frequently calls In the aid of mathematical language and argument. With these latter, the student supposed to be sufficiently familiar; there remains, therefore, nothing to be explained, but the mechanical terms. Some of the more obvious of these have already, of necessity, been used, and might be considered as sufficiently understood Yet a few words of definition may not be am:«

Matter.—Every thing known by the senses.

Body.—Every quantity of matter.

Atoms.—The mechanically indivisible parts of matter.

Mass.—The quantity of matter. This is tw ordinary definition; but mechanicians expres mass of matter by a formula, too abstract to fine place here.

T'ohtme.—The space occupied by any quantity of matter.

Force.—Any cause capable of producing or modifying motion. An ivory ball laid upon a table, Fig. 46, is struck with it cue; the motion

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The above diagram is intended to represent the mode of formation of anobula of this kind.

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moving, exerts a force which modifies the direction of the original motion. Force, therefore, 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 direction and point of application ; and the relatin' 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 C D, Fig. 48, be tbo two forces acting on the body M ; and A R = IS x C D. Now A K is made to show the

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subject to the control of xathematical 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.

Requirements.—The requirement of the force is its relation to all position* 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 P acting upon the body A, Fig. 47, is represented in direction and intensity by this P A; and its requirement regarding the position a b is indicated by the perpendicular P p on a b 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 position ab? It wants to place the body at a certain perpendicular distance from a b. It may move the body to P, but if not allowed, its want (requirement) is still satisfied if the body at another point N be at the same distance from a b as if at P—i.e., if N « = Pp.

Resultant. Signiñes the effect which any *■ / a . so number of forces do

"■"j, produce, or could produce if allowed to act. In Fig. 50, let P and Q 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 follow the line A E. This line, or the force B, is the resultant of the two forces P and Q.

Components.—The forces which produce the effect. Thus, P and Q are the component, Fig. 60.

Composition.—The replacing two or more forces by one resultant. P and Q replaced by B.

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 lest 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.

JUotien.—The exertion of the power of moving, or the being forced to change position in space.

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PART II.

Mechanics Op 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, etc. This part will contain three chapters, one for each of the states. The first of the three, of "solids," treats what is usually under

stood by mechanics. It is much longer than the other two, as therein are established the principles of the science in its widest sense. The second and third chapters show how far these 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.

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" to 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 introduced here.

Section I. 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 marked in the solid, when contrasted with the liquid and gaseous state. And these ere—figure and gravitation. Some solids, as stated, affect certain definite forms; but all are indifferent to any form which external causes may give them. The most complicated crystal, cut into a simple form, retains this form. On the other side, many solid, or rather all that deserve to be called solids, 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 recipient which contains it. Still, it is known, Tresca of Paris, has proved it by many and long-continued experiments, that metals (cold) can be made to flow, somewhat like liquids.

Gravitation has the same effect on solids no matter in what position they bo placed. A long rod of glass, for instance, resting on a table, on a single point, will press equally, whether laid horizontal or vertical. Again, a disc and rod of iron, Fig. 51, will press no more upon the pan of a r a c 5/ balance, when the rod

is (screwed) perpendicular a, than when it lies horizontal b. The reason of remarking these two wellknown facts will appear in the Mechanics of liquids.

Other properties might be mentioned, of which the effects in I =tbe 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,

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that, in this section, the object always is, to de termine how equilibrium may be prod need, or as it is usually expressed the "conditions of equilibrium" of forces applied to a body.

Forces can be applied to a body in two ways only ; Ie parallel, 2e parallel to a point, or its equivalent. Ag regards the action of the forces this can be shown experimentally. In Fig. 52 let the rods P and Q represent two forces parallel acting upon the body A at the pivots a a. Move the code 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. For it 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. M, under the action of three forces PQK in equilibrium. The forces P and Q hang parallel from the corners; It 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 a a, and the weight R adjusted to suit the new conditions of equilibrium, which was disturbed; _why ?—no matter, just for the moment. Now, whether the forces P and Q be kept pinned to the corners, or allowed to »ct along the prolongation of their direction at B, the state of equilibrium remains. Therefore the action of tua forces P and Q is not changed by the transfer. It is clear, moreover, that the point B might be anywhere outside the limits of the body A, provided the condition of rigid connection, between the point B and the body A be secured. To the body A, Fig. 54, two forces are applied 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 -f equilibrium. All t!" principles of Statics f" the conditions of "uni- librium") are therefore to be classed under the two titles " forces applied to a point ;'' "forces applied parallel to a rod."

Into a course of Statics is always introduced the^explanation of the so-called, " machines," mechanical powers. And though, in practice, they ore much used for motion, and are often exhibited 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.)

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GALVANIC BATTEBIES.»

THERE have been within the past few years a variety of batteries invented in France; some of them have been used to a very considerable 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 inGanot's physics 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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