ing the useful portion of the seed itself. This starch cells; but all of them ought to be present 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 -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 cecidedly hold We that this admixture is not a chemical adjustment 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. iv W (Continued from page 274.) ITH the view of placing the question 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 2155lb. 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 per square inch of the same, is (say) 13lb. ing in the British Islands to a depth of 4000 feet 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 Deep mining has been carried on much more valve friction may become considerable, even extensively in Belgium than in England, there three times higher than stated. But worse than being only twelve pits of a greater depth than the friction itself are its results—namely, undue 1500ft. in the latter country, as compared with wear, scoring, &c., causing extra loss in fuel by sixty-eight in the former. The deepest coal mine leakage of fresh steam into the cylinder and the in the world was probably that of Simon Lambert, exhaust. The piston friction co-efficient does in Belgium, which had attained the great depth of 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 has 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 1840 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 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 adinit. 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 beavy 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 × 065 three mines of a high elevation was 1 for every 71-6ft., whilst the increase for three minesat some distance below the level of the sea was 1 for every 62-3ft. Friction of Crosshead Pin.-2in. diameter, connecting rod mean pressure ns. me, say 2001b., =475, friction, co-efficient, 085;1 × crank 4 X 4.75 105, which is a factor used several times; Mr. MFarlane Gay's abbreviated formulæ used; then with 110 revolutions 200 x 65 x 1 611ft. lbs. == 12 3.6ft. lbs. only. 0 X 07 X 7854 4.75 X277= 10.2 X 1·03 X X 110 = 7·63 × S 33-3ft. lbs. Friction of Crank Shaft.-Here we must add the weight of crank, fly-wheel, &c. (say 90015.), 9-425 then 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 × 103-9 × 277 = 50848ft. lbs., out of which 49548ft. lbs. for piston and packings, and 13001b. 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 508,180, 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 62518, very nearly 4-5ths are absorbed by piston and packings. Here we are at the root of the matter, and if we want to economise, here is a wide field for every foot pound lost in fricion means a direct loss in money or fuel. It should be Loticed that in this case the piston = The following are the results:-With st am in The waste heat of the uptake is often used for (To be continued.) The experiments relating to the underground temperature of the air at the Rosebridge Colliery showed an increase in the temperature of the air in passing from the downcast to the upcast shaft of from 55° to 63°; the air passing through workings the temperature of which was 789, and the normal temperature of the coal being 93.5. The 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°. 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. It would appear, from the contradictory results of the experiments relating to the temperature of different minerals, that no rule 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 minerals. Judging from the statistics of the past few years, the production of the British coal fie ds could not be considered to increase annually in a constantly In regard to the increase of temperature with increasing ratio, as had been surmised, but might the distance from the surface, a careful combe estimated at an average "output" of 105 mil-parison of all the experiments quoted, and especi lions of tons yearly. Estimating the coal remain ally of those taken at a greater depth than 2000ft led to the conclusion, that as far as could be judged | channel, as the length of the perimeter, and as Of the three modes by which heat was lost by one substance and absorbed by another, viz., radiation, conduction, and convection, the only infiuence 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, dne 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 59°. 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 average about 15° for every 500 yards. The question of ventilating a mine 7000ft. deep, to an extent sufficient to absorb the heat emitted by strata having a normal assumed temperature of 1769, 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 62, and the normal temperature would be found as far as 12ft. from the surface of the mineral. 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 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 compressel air, and the sudden 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 could be anticipated, since the quantity of air exhausted bore so small a proportion to an ordinary current of air, that the effect on the temperature was only to be observed locally, and 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 substances, nor the employment of the electric light, nor the use of cold water and ice, could be anticipated to have any effect worthy of note. The hygrometrical 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 case 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 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 "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 and the expenditure of the times. THE AERONAUTICAL SOCIETY. THE pro was calculated to be 40 days. C. The total Teneral meeting of members of the above from 96° to 141°, was found to be 14:04 tons every twenty-four hours. The laws upon which the amount of power necessary to produce a certain quantity of air under every condition were stated to be as follows:-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 a sufficient initial velocity to create in the air a solid resistance. His suggestion was that man should be taught to fly as youths were taught to swim in Germany-namely, by being suspended by a rope, when he could in safety make experi ments with an artificial flying apparatus. He was 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 air upon which it worked without making progress, but if it were 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 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 aërostation.-The Chairman said that the last speaker had really touched the true principle. If ever they obtained perfect aërostation it must 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 thought it would be worthy work for the society to test his theory by experiment. After some purely scientific discussion on the anemometer, a gentleman, suggested that a light canoe working on the water would be a better medium of experiment with the propellers than the velocipede.-Mr. Louis Olrick, consulting engineer, observed that the inventors of the sewing machine had not succeeded by endeavouring to imitate the old sewing process. They started on an entirely new principle, and aëronants, 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 acrostation. What they wanted was patient experiment, and experiments cost money; aërostation 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 measuring the pressure of the air was exhibited, and explained by Dr. Smith, and very much commended by the Chairman. 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 suu itself is a nebulous star, or that it is enveloped by matter extending to an immense distance beyond its visible photosphere. The phenomenon matter passed through by the earth in its revoluof the zodiacal light, the bands of meteoric tion 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 sup pose for a moment that solid matter exists without being enveloped by gas, it will immediately itself. This atmosphere will be very rare and 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 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. Mr. Walter Clure then proceeded to give his unenveloped by gas would not acquire an atmos Graham has found that meteoric matter which Master. We conclude, then, that the sun is surrounded by an envelope of gas, which is not a true solar 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 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 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. and the light shading denote, respectively, the large mass of rarer gas and the small mass of 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 remains, 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 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 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 C D, 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 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. FIG. 49 P stood by mechanics. It is much longer than the 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 for immense importance in the two other states, 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 p direction and intensity by the PA; and SECTION I. represent two forces from the 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. position a b? 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 MATTER, GENERAL PROPERTIES. Solid bodies may be said to possess all the general properties of matter in a greater or less a a, and the weight R adjusted to suit the new degree. There are two however, which are very 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 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 when cold, no matter what the shape of the regreat force. A mass of iron, has the same form of 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 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 ab as if at P-i.e., if N n = Pp. Resultant.Signifies the effect which any FIG. 50 R number of forces do as absolute rest. That, in its strict sense, would of are sufficient to direct the mind to other facts, 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, | FIG. 54. B 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 of equiprinciples of Statics (the conditions of equilibrium") are therefore to be classed under the two titles "forces applied to a point;"' "forces applied parallel to a rod." librium. All Into a course of Statics is always introduced the explanation of the so-called, "machines," mechanical powers. And though, in practice, they are 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 GALVANIC BATTERIES.* HERE 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 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." |