(8.)

(8.)Fluid bodies are of two kinds, inelastic fluids, or liquids, and elastic fluids, or gases. Of the former of these classes, water is the most familiar example, and of the latter, air.

These two species of fluids are each distinguished by peculiar mechanical properties.

Fig. 3.

(9.)The constituent particles of a liquid are distinguished from those of solids by having little or no coherence; so that unless the mass be confined by the sides of the vessel which contains it, the particles will fall asunder by their gravity. A mass of liquid, therefore, unlike a solid, can never retain any particular form, but will accommodate itself to the form of the vessel in which it is placed. It will press against the bottom of the vessel which contains it with the whole force of its weight, and it will press against the sides with a force proportional to the depth of the particles in contact with the sides measured from the surface of the liquid above. This lateral pressure also distinguishes liquids from solids. Let us take for illustration the case of a square or a cubical vessel,A B C D,fig.3.If a solid body, such as a piece of lead, be cut to the shape of this vessel, so as to fit in it without pressing with any force against its sides, the mechanical effect which would be produced by it when placed in the vessel, would be merely a pressure upon the bottom,B C, the amount of which would be equal to the weight of the metallic mass. No pressure would be exerted against the sides; for the coherence of the particles of the solid maintaining them in their position, the removal of the sides would not subject the solid body contained in the vessel to any change.

Now let us suppose this solid mass of lead to be rendered liquid by being melted. The constituent particles will then be deprived of that cohesion by which they were held together; they will accordingly have a tendency to separate, and fall asunder by their gravity, and will only be prevented from actually doing so by the support afforded to them by the sides,[Pg026]A B,D C, of the vessel. They will therefore produce a pressure against the sides, which was not produced by the lead in its solid state. This pressure will vary at different depths: thus a part of the side of the vessel atPwill receive a pressure proportional to the depth of the pointPbelow the surface of the lead. If, for example, we take a square inch of the inner surface of the side of the vessel atP, it will sustain an outward pressure equal to the weight of a column of lead having a square inch for its base, and a height equal toP A. And, in like manner, every square inch of the sides of the vessel will sustain an outward pressure equal to the weight of a column of lead having a square inch for its base, and a height equal to the depth of the point below the surface of the lead.

(10.)We have here proceeded upon the supposition that no force acts on the upper surfaceA Dof the lead. If any force pressesA Ddownwards, that force would be transferred to the bottom by the lead, and would produce a pressure on the bottomB Cequal to its own amount in addition to the weight of the lead; and if the lead were solid, this would be the only additional mechanical effect which such a force acting on the surfaceA Dof the lead would produce. But if, on the other hand, the lead were liquified, then the force now adverted to, acting on the surfaceA D, would not only produce a pressure on the bottomB C, equal to its own amount in addition to the weight of the lead, but it would also produce a pressure against every part of the sides of the vessel, equal to that which it would produce upon an equal magnitude of the surfaceA D.

Thus if we suppose any mechanical cause producing a pressure on the surfaceA Damounting to ten pounds on each square inch, the effect which would be produced, if the lead were solid, would be an additional pressure on the baseB Camounting to ten pounds per square inch. But if the lead were liquid, besides this pressure on each square inch of the baseB C, there would likewise be a pressure of ten pounds on every square inch of the sides of the vessel.

All that has been here stated with respect to a square or a cubical vessel will be equally applicable to a vessel of any other form.[Pg027]

(11.)The second class of fluids are distinguished from liquids by the particles not merely being destitute of cohesion, but having a tendency directly the reverse, to repel each other, and fly asunder with more or less force. Thus if a vessel, such as that represented infig.3., were filled with a fluid of this kind, being open at the top, and not being restrained by any pressure incumbent upon it, the particles of the fluid would not rest in the vessel by their gravity, as those of the liquid would do; but they would, by their mutual repulsion, fly asunder, and rise out of the vessel, as smoke is seen to rise from a chimney, or steam from the spout of a kettle. Let us suppose, then, that the vessel in which an elastic fluid is contained is closed on every side by solid surfaces. In fact, let us imagine that the square or cubical vessel represented infig.3.is closed by a square lid at the topA D, having contained in it an elastic fluid, such as atmospheric air.

If such a cover, or lid, had been placed upon a liquid, the cover would sustain no pressure from the fluid, nor would any mechanical effect be produced, save those already described in the case of the open vessel; but when the fluid contained in the vessel is elastic, as is the case with air, then the elasticity (by which name is expressed the tendency of the particles of the fluid to fly asunder) will produce peculiar mechanical effects, which have no existence whatever in the case of a liquid.

It is true that, supposing the fluid to be air or any other gas or vapour, a pressure will be produced upon the bottomB Cof the vessel equivalent to the weight of such fluid, and lateral pressures will be produced on the different points of the sides by the weight of that part of the fluid which is above these points; but gases and vapours are bodies of such extreme levity, that these effects due to their weight are neglected in practice.

Putting, then, the weight of the air contained in the vessel out of the question, let us consider the effect of its elasticity. If the vessel, as already described, be supposed to contain atmospheric air in its ordinary state, the tendency of the constituent particles to fly asunder will be such as to produce on every square inch of the inner surface of the vessel[Pg028]a pressure amounting to fifteen pounds; this pressure being, as already stated, quite independent of the weight of the air. In fact, this pressure would continue to exist if the air contained in the vessel actually ceased to have weight by being removed from the neighbourhood of the earth, which is the cause of its gravity.

(12.)Different gases are endowed with different degrees of elasticity, and the same gas may have its elasticity increased or diminished, either by varying the space within which it is confined, or by altering the temperature to which it is exposed.

If the space within which an elastic fluid is enclosed be enlarged, its elasticity is found to diminish in the same proportion. Thus if the air contained in the vesselA B C D(fig.3.) be allowed to pass into a vessel of twice the magnitude, the elasticity of the particles will cause them to repel each other, so that the same quantity of air shall diffuse itself throughout the larger vessel, assuming double its former bulk. Under such circumstances, the pressure which it would exert upon the sides of the larger vessel would be only half that which it had exerted on the sides of the smaller vessel. If, on the other hand, it were forced into a vessel of half the magnitude ofA B C D, as it might be, then its elasticity would be double, and it would press on the inner surface of that vessel with twice the force with which it pressed on that of the vesselA B C D.

This power of swelling and contracting its dimensions according to the dimensions of the vessel in which it is confined, or to the force compressing it, is a quality which results immediately from elasticity, and is consequently one which is peculiar to the gases or elastic fluids, and does not at all appertain to liquids. If the liquid contained in the vesselA B C Dwere transferred to a vessel of twice the magnitude, it would only occupy half the capacity of that vessel, and it could not by any means be transferred, as we have supposed the air or gas to be, to a vessel of half the dimensions, since it is inelastic and incompressible.

(13.)The elasticity of gases is likewise varied by varying the temperature to which they are exposed; thus, in general,[Pg029]if air or any other gas be augmented in temperature, it will likewise be increased in elasticity; and if, on the other hand, it be diminished in temperature, it will be likewise diminished in its elastic force. The more heated, therefore, any air or gas confined in a vessel becomes, the greater will be the force with which it will press on the inner surface of that vessel, and tend to burst it.

(14.)The same body may, by the agency of heat, be made to pass successively through the different states of solid, liquid, and gas, or vapour. The most familiar and obvious example of these successive transitions is presented by water. Exposed to a certain temperature, water can only exist as a solid; as the temperature is increased, the ice, or solid water, is liquefied; and by the continued application of heat, this water again undergoes a change, and assumes the form, and acquires the mechanical qualities, of air or gas: in such a state it is calledSTEAM.

This is a common property of all liquids. If they be exposed for a sufficient length of time to a sufficient degree of heat, they will always be converted into elastic fluids. These are usually distinguished from air and other permanent gases, which never are known to exist in the liquid form, by the termvapour, by which, therefore, must be understood an elastic fluid which at common temperatures exists in the liquid or solid state; bysteamis expressed the vapour of water; and bygases, those elastic fluids which like air are never known—at least, under ordinary circumstances—to exist in any other but the elastic form.

(15.)When a liquid is caused, by the application of heat, to take the form of an elastic fluid, or is evaporated, besides acquiring the property of elasticity, it always undergoes a considerable change of bulk. The amount of this change is different with different liquids, and even with the same liquid it varies with the circumstances under which the change is produced.

(16.)When water is evaporated under ordinary circumstances,—that is, when exposed to no other external pressure than that of the atmosphere,—it increases its volume about seventeen-hundred-fold. Thus a cubic inch of liquid[Pg030]water would form about seventeen hundred cubic inches of common steam. If, however, the water be confined by a greater pressure than that produced by the common atmosphere, then the increase of volume which takes place in its evaporation would be less in proportion.

These important physical circumstances are now only indicated in a general way. As we proceed with our account of the invention and improvement of the steam engine, they will be developed more fully and accurately.

(17.)After duly considering what has been just explained, no difficulty will be found in comprehending the principles on which the first rude attempts at the mechanical application of steam already stated depend. In the apparatus ascribed toHeroof Alexandria, the elasticity of the vapour contained in the arms of the revolving ball causes that vapour to issue from the lateral orifices in the arms, such as that ofG,fig.1.As these orifices, however, are exposed to the common atmosphere pressing inwards with a force, the mean amount of which has been stated to be about fifteen pounds per square inch, it follows that the steam cannot escape from these orifices until its pressure or elasticity exceeds this amount, and that when it does, the force with which it will so escape will be the excess of its elasticity above that of the atmosphere; and it is the reaction produced by this difference of pressure, causing the arms to recoil, which will give motion to the machine.

In the case of the apparatus ofDe Caus(5.), the heat of the fire acting on the vesselD C(fig.2.) will raise the temperature of the water contained in it, and also of the air confined within it above the surface of that water. This air, as it is increased in temperature, will also increase in elasticity; it will therefore press on the surface of the water with increased force, and will gradually force the water upwards in the tube; and this effect would continue until all the water in the vessel would be forced up the tube.

But at the same time that the heat acting on the vessel increases the temperature of the air above the water, it also produces a partial evaporation of the water, so that more or less steam is mixed with the air in the vessel above the surface[Pg031]of the water; and this steam possessing elasticity, unites with the air in pressing on the surface of the water, and in raising it in the tube.

Figs. 4, 5, and 6.

Let us now revert to the brief account of the engine of the Marquis of Worcester, described in "The Century of Inventions." We collect from that description that the vessel in which the water was evaporated was separate from those which contained the water to be elevated; also that there were two vessels of the like description, the contents of which were alternately elevated by the pressure of the "water rarefied by the fire;" in other words by steam; and that the water was raised in an uninterrupted stream, by the management of two cocks communicating with these vessels and with the boiler. The following is such an apparatus as would answer this description. LetE(fig.4.) be the vessel containing the water to be evaporated, placed over a proper furnaceA; letSbe a pipe to allow the steam produced from the boiling water inEto pass into the vessels where its mechanical action is required. LetRrepresent a cock or regulator, having in it a curved passage, leading fromSto the tubeT, when the lever or handleLis in the position represented by the cut; but leading to the tubeT′, when the leverLis turned one quarter of a revolution to the right, as represented infig.5.By the shifting of this lever, therefore, the steam pipeSmay be made to communicate alternately with the tubesTandT′. The tubesTandT′are carried respectively to two vesselsVandV′, which are filled with the water required to be raised. In these[Pg032]vessels tubes enter atCandC′, descending nearly to the bottom: these tubes have valves atBandB′, opening upwards, by which water will be allowed to pass into the vertical tubeF, but which will not allow it to return downwards, the valvesBandB′being then closed by the weight of the water above them.

LetG G′be a pipe entering the sides of the vesselsVandV′, for the purpose of filling them with the water to be raised: letKbe a cock having a curved passage similar to the cockR, and leading to a tube by which water is supplied from the reservoir or other source from which the water to be raised is drawn. When the cockKis placed as represented infig.4., the water from the reservoir will flow through the curved passage in the cockKinto the tubeG′, and thence into the vesselV′; but when this cock is turned one quarter round, by shifting the lever to the left, it will take the position represented infig.6., and the water will flow through the curved passage into the tubeG, and thence into the vesselV. Let us now suppose the vesselValready filled with water to be elevated, and the vesselV′to have discharged its contents. The cockRis turned, so as to allow the steam generated in the boilerEto pass into the tubeT, and thence into the upper part of the vesselV, while the cockKis turned so as to allow the water from the reservoir to pass into the tubeG′, and thence into the vesselV′. The steam collecting in the upper part of the vesselV′presses with its elastic force on the surface of the water therein, and forces the water upwards in the tubeC; it passes through the valveB, which it opens by the upward pressure received from the action of the steam, and thence into the tubeF, its descent into the tubeC′being prevented by the valveV′, which can only be opened upwards. As the steam is gradually supplied from the boilerE, the water in the vesselVis forced up the tubeC, through the valveB, and into the tubeF, until all the contents of the vesselVabove the lower end of the tubeChave been raised. In the meanwhile, the vesselV′has been filled with water, through the cockK: when this has been accomplished, the man who attends the machine shifts the cocksRandK, so as to give them the position represented infig.5.andfig.6.[Pg033]In this position, the steam from the boiler, being excluded from the tubeT, will be conducted to the tubeT′, and thence to the vesselV′, while the water from the reservoir will be excluded from the tubeG′, and conducted through the tubeGto the vesselV. The vesselVwill thus be replenished and, by a process similar to that already described, the contents of the vesselV′will be forced up the tubeC′, through the valveB′, and into the tubeF; its descent into the tubeCbeing prevented by the valveB, which will then be closed. After the contents of the vesselV′have thus been raised, and the vesselVreplenished, the two cocksRandKare once more shifted, and the contents ofVraised whileV′is replenished, and so on.

Fig. 4, 5, and 6.

If, having comprehended the apparatus here described, the reader refers to the description of the Marquis of Worcester's machine, he will find that all the conditions therein laid down are fulfilled by it. One vessel (E) of "water rarefied by fire" may by such means "drive up forty (or more) of cold water; and the man that tends the work has but to turn two cocks, that one vessel (V) of water being consumed, another (V′) begins to force and refill with cold water, and so on successively, the fire being tended and kept constant; which the self-same person may likewise abundantly perform, in the interim between the necessity of turning the said cocks."

On comparing this with the contrivance previously suggested by De Caus, it will be observed, that even if De Caus[Pg034]knew the physical agent by which the water was driven upwards in the apparatus described by him, still it was only a method of causing a vessel of boiling water to empty itself; and before a repetition of the process could be made, the vessel should be refilled, and again boiled. In the contrivance of Lord Worcester, on the other hand, the agency of the steam was employed in the same manner as it is in the steam engines of the present day, being generated in one vessel, and used for mechanical purposes in another. Nor must this distinction be regarded as trifling or insignificant, because on it depends the whole practicability of using steam as a mechanical agent. Had its action been confined to the vessel in which it was produced, it never could have been employed for any useful purpose.

Although many of the projects contained in Lord Worcester's work were in the highest degree extravagant and absurd, yet the engine above described is far from being the only practicable and useful invention proposed in it. On the contrary, many of his inventions have been reproduced, and some brought into general use since his time. Among these may be mentioned, stenography, telegraphs, floating baths, speaking statues, carriages from which horses can be disengaged if unruly, combination locks, secret escutcheons for locks, candle moulds, the rasping mill, the gravel engine, &c.

Sir Samuel Morland, 1683.

(18.)Sir Samuel Morland was the son of a baronet of the same name, who had received his title at the restoration for some services to the royalist party, performed by him during the wars of the Commonwealth. He appears to have devoted much attention to mechanics, in which he attained some celebrity. He was the reputed inventor of several ingenious contrivances, such as the drum capstan for ships, the plunger pump, &c. He also investigated various questions in acoustics, and among others, the determination of the best form for the speaking-trumpet.

In 1680, Sir Samuel Morland was appointed Master[Pg035]of the Works to Charles II., and in the following year was sent to France, to execute some waterworks for Louis XIV. In 1683, while in France, he wrote in the French language, a work entitled "Elevation des Eaux par toute sorte de Machines, reduite à la Mesure, au Poids et à la Balance. Presentée à sa Majesté très Chrestienne, par le Chevalier Morland, Gentilhomme Ordinaire de la Chambre Privée, et Maistre des Méchaniques du Roi de la Grande Brétagne, 1683." This book is preserved in manuscript in the Harleian Collection in the British Museum. It is written on vellum, and consists of only thirty-eight pages. It contains tables of measures and weights, theorems for the calculation of the volumes of cylinders, the weights of columns of water, the thickness of lead for pipes, and is concluded by a chapter on steam, consisting of four pages, of which the following is a translation:—

"The principles of the new force of fire invented by Chevalier Morland in 1682, and presented to His Most Christian Majesty in 1683:—

"'Water being converted into vapour by the force of fire, these vapours shortly require a greater space (about 2000 times) than the water before occupied, and sooner than be constantly confined would split a piece of cannon. But being duly regulated according to the rules of statics, and by science reduced to measure, weight, and balance, then they bear their load peaceably (like good horses), and thus become of great use to mankind, particularly for raising water, according to the following table, which shows the number of pounds that may be raised 1800 times per hour to a height of six inches by cylinders half filled with water, as well as the different diameters and depths of the said cylinders.'"

There is nothing in the description here given which can indicate the form of the machine by which Morland proposed to render the force of steam a useful mover. It is, however, remarkable, that at this early period, before experiments had been made on the expansion which water undergoes in evaporation, he should have given so near an approximation to[Pg036]the actual amount of that expansion. It is scarcely supposable that such an estimate could be obtained by him otherwise than by experiment.

The work containing the above description was not printed; but a work bearing nearly the same title, containing, however, no mention of the force of steam, was published by him in Paris in the year 1685. In this he describes various experiments made by him at St. Germains on the weight of the water of the Seine, and gives weights of the columns of water, the contents of cylinders, &c.

Soon after the publication of this work, Morland returned to England, and resided near the court till his death. The celebrated John Evelyn mentioned having paid a visit to him at his house at Hammersmith, in 1695, when he had become aged and blind, but was still remarkable for his mechanical ingenuity. "On the 25th of October," says Evelyn, "the Archbishop and myself went to Hammersmith to visit Sir Samuel Morland, who was entirely blind; a very mortifying sight. He showed us his invention of writing (short-hand), which was very ingenious; also his wooden kalendar, which instructed him all by feeling; and other pretty and useful inventions of mills, pumps, &c.; and the pump he had erected, that serves water to his garden and to passengers, with an inscription, and brings from a filthy part of the Thames near it a most perfect and pure water."[3]

He died at Hammersmith, in January 1696; and before his death, as a penance for his past life, was guilty of the eccentricity of burying in the ground six feet deep a great collection of music which he possessed.[4]

Denis Papin, 1688.

(19.)Denis Papin, a native of Blois in France, and professor of mathematics at Marbourg, is the name which stands next recorded in the progressive invention of the steam engine. To this philosopher is due the discovery of one of the qualities of steam, to the proper management of which is owing much of the efficacy of the modern steam engine.[Pg037]

Papin was born at Blois in France. He devoted his youth to the study of medicine, in which he took a degree at Paris. The revocation of the Edict of Nantes having driven him into exile, he went to England, where the celebrated Boyle associated him in several of his experiments with the air-pump, and caused him to be elected a fellow of the Royal Society in 1681. Having been invited to Germany by the Landgrave of Hesse, he discharged during several years the duties of professor of mathematics at the university of Marbourg, where he died in 1710. Notwithstanding his discoveries respecting the agency of steam, he never received any mark of distinction in his own country. The truth is, the importance and value of these investigations were not apparent until long afterwards.

This philosopher conceived the idea of producing a moving power by means of a piston working in a cylinder, in the manner which we shall now briefly explain.

Fig. 7.

LetA B(fig.7.) be a cylinder open at the top, and let a pistonPbe fitted into it, so as to move in it air tight. At the bottom of the cylinder suppose an opening provided, which can be closed at pleasure, by a stop-cock, or otherwise, so that the communication may be opened and closed at will between the interior of the cylinder and the external air. This stop-cock being opened, let the piston be drawn upwards till it reach the top of the cylinder. Let the stop-cock at the bottom be then removed, and imagine that some means can be supplied by which the air within the cylinder can be suddenly annihilated. The piston, now at the top, will have above it the pressure of the atmosphere; and having no air below, it will be resisted in its descent by no force save that arising from its friction with the cylinder. If, then, the force of the air above the piston be greater than the resistance arising from this friction, the piston will descend with the excess of this force, and will continue so to descend until it reach the bottom of the cylinder. Having attained that position, let us[Pg038]suppose the stop-cock in the bottom opened, so as to allow the external air to pass freely below the piston. The piston may now be drawn to the top of the cylinder again, offering no resistance save that of its weight, and its friction with the cylinder. Having reached the top of the cylinder once more, let the stop-cock be closed, and the air included within the cylinder once more annihilated. A second descent of the piston will take place, with the same force as before, and in like manner the process may be continued indefinitely.

Now, if it should appear that means could be provided suddenly and repeatedly to annihilate the air within the cylinder, and that the pressure of the atmosphere above the piston should exert a force compared with which the weight of the piston and its friction are trifling, it is evident that a moving power would be obtained which would be capable, by proper mechanism, of being applied to any useful purpose, but which would more especially be applicable to the working of pumps, the motion of which corresponds with that which has been just ascribed to the piston in the cylinder. Such were the first ideas of Papin. But in order to enable those who are not conversant with physical science fully to appreciate their importance, it will be necessary here to explain some of the mechanical properties of atmospheric air.

(20.)The atmosphere is the thin, transparent, colourless, and therefore invisible, fluid in which we live and move, which by respiration sustains animal life, and is otherwise connected with various important functions of organised matter. This fluid is so light and attenuated, that it might at first be doubted whether it be really a body at all; and, indeed, the name expressing incorporeal beings,spirit, is a word in its origin signifyingair.[5]The air, however, is light only as compared with other material substances, which exist in a more condensed state: it possesses the quality of weight as absolutely as the most solid and massive bodies in nature, and to render this quality manifest, it is only necessary to submit a sufficient quantity of air to any of the usual tests of gravitation.[Pg039]

A direct demonstration of this may be given by the following experiment:—On the mouth of a flask let a stop-cock be fastened so as to be air-tight. The interior of the flask may then be put into free communication with the external air, or that communication may be cut off at pleasure, by opening or closing the stop-cock. If a syringe be applied to the mouth of the flask, the stop-cock being open a part of the air contained in it may be drawn out. After this, the stop-cock being closed, and the syringe detached, let the flask be placed in the dish of a good balance, and accurately counterpoised by weights in the other dish. This counterpoise will then represent the weight of the flask, and of the air which has remained in it. If the stop-cock be now opened, air will immediately rush in, and replace that which the syringe had withdrawn from the flask; and immediately the dish of the balance containing the flask will sink by the effect of the weight of the air thus admitted into the flask.

If the weight of quantity of air so small as to be capable of being withdrawn by a syringe from an ordinary flask be thus of sensible amount, it may be easily imagined that the vast mass of atmosphere extending from the surface of the earth upwards, to a height not ascertained with precision, but certainly not being less than thirty miles, must be very considerable. Such a force, pressing as it must constantly do, upon the surfaces of all bodies, whether solid or fluid, and resisting and modifying their movements, would play an important part in all mechanical phenomena; and it is, therefore, not sufficient merely to have recognised its existence, but it is most needful to measure its amount with that degree of certainty and precision, which will enable us to estimate its effects on those phenomena which we shall have to investigate.

(21.)The amount of the pressure of the atmosphere on each square inch of horizontal surface on which it rests, is obviously the weight of the column of air extending from that square inch of surface upwards to the top of the atmosphere. This force is measured by the following means:—

Figs. 8., 9.

Take a glass tube,A B(fig.8.), above 32 inches long, open at one endA, and closed at the other endB, and let it[Pg040]be filled with mercury (quicksilver). Let a glass vessel or cisternC, containing a quantity of mercury, be also provided. Applying the finger atA, so as to prevent the mercury in the tube from falling out, let the tube be inverted, and the end, stopped by the finger, plunged into the mercury inC. When the end of the tube is below the surface of the mercury inC(fig.9.), let the finger be removed. It will be found that the mercury in the tube will not, as might be expected, fall to the level of the mercury in the cisternC, which it would do were the endBopen, so as to admit the air into the upper part of the tube. On the other hand, the levelDof the mercury in the tube will be nearly 30 inches above the levelCof the mercury in the cistern.

The cause of this effect is, that the weight of the atmosphere rests on the surfaceCof the mercury in the cistern, and tends thereby to press it up, or rather to resist its fall in the tube; and as the fall is not assisted by the weight of the atmosphere on the surfaceD(sinceBis closed), it follows, that as much mercury remains suspended in the tube above the levelC, as the weight of the atmosphere is able to support.

If the section of the tube were equal to the magnitude of a square inch, the weight of the column of mercury in the tube above the levelCwould be exactly equal to the weight of the atmosphere on each square inch of the surfaceC.

(22.)If such an apparatus be observed from time to time, it will be found that the column of mercury sustained in the tube will be subject to variation between certain limits, never falling below twenty-eight inches, and never rising above thirty-one inches. This variation of the mercurial column is produced by a corresponding variation in the weight of the atmosphere.

If the apparatus be transported to any height above its ordinary position, it will have a less quantity of atmosphere above it, and therefore the surface of the mercury in the cistern will be pressed by a less weight, and consequently the[Pg041]column of mercury will fall proportionally. In virtue of this effect, such an instrument has been rendered a means of measuring heights, such as the heights of mountains, the ascents of balloons, &c. &c.

(23.)If a proper scale be attached to the tube containing the mercurial column, showing the absolute height of the column sustained at any time, and indicating its changes of height, the instrument becomes aBarometer.

Two cubic inches of mercury weigh very nearly one pound avoirdupois.[6]Hence, when the barometric column measures thirty inches, the weight of the atmosphere resting on each square inch of surface is about fifteen pounds.

(24.)It is an established property of fluids, that they press equally in all directions; and air, like every other fluid, participates in this quality. Hence, it follows, that when the downward pressure or weight of the atmosphere is fifteen pounds on the square inch, the lateral, upward, and oblique pressures are of the same amount. But, independently of the general principle, it may be satisfactory to give experimental proof of this.

Fig. 10.

Let four glass tubes,A,B,C,D(fig.10.), be constructed of sufficient length, closed at one end,A,B,C,D, and open at the other. Let the open ends of three of them be bent, as represented in the tubesB,C,D. Being previously filled with mercury, let them all be gently inverted, so as to have their closed ends up, as here represented. It will be found that the mercury will be sustained in all, and that the difference of the levels in all will be the same.[7]Thus, the mercury is sustained inAby the upward pressure of the atmosphere; inB, by its horizontal or lateral pressure; inC, by its downward pressure;[Pg042]and inD, by its oblique pressure: and, as the difference of the levels is the same in all, these pressures are exactly equal.

(25.)The same arrangement by which the pressure of the atmosphere is measured by a mercurial column of equivalent weight, also supplies the means of measuring the pressure or elasticity of atmospheric air, or any other gas or vapour, whether in a more or less compressed or rarefied state; and as instruments constructed on this principle are of considerable use in steam engines, we shall take this occasion to explain their principle and application.

In the experiments described in (21), the spaceD Bin the top of the barometer-tube, from which the mercury descended, is a vacuum. If, however, it were occupied by a quantity of air in a rarefied state, or any other gas or vapour, such gas or vapour would press on the surface of the mercury atD, with a force determined by its elasticity. In that case, the atmospheric pressure acting on the surface of the mercuryCin the cistern, would be balanced by the combined forces of the weight of the mercurial column sustained in the tube, and the elasticity of the gas or vapour in the upper part of it. Now if we know the actual amount of the atmospheric pressure,—that is to say, the height of the column of mercury which it would be capable of sustaining,—we should then be able to determine the pressure of the rarefied air in the spaceC D.

For example, let us suppose that the barometric column, whenB D(fig.9.) is a vacuum, measures thirty inches: the atmospheric pressure, therefore, would be equal to the weight of a column of mercury of that height. Let us suppose that the elasticity of the gas or vapour occupying the upper part of the tubeD Bcauses the column to fall to the height of twenty-six inches: it is evident, then, that the pressure of the air in the top of the tube would be equal to the weight of a column of mercury of four inches. In fine, to determine the pressure of the rarefied gas or vapour in the top of the tube, it is only necessary to observe the difference between the height of the column of mercury actually sustained in the tube, and the column sustained at the same time and[Pg043]place in a common barometer: the difference of the two will be the column of mercury whose weight will represent the pressure of the vapour or gas in the top of the tube.

(26.)Whenever the air contained in any vessel or other enclosed space has by any means had its pressure reduced so as to be rendered less than that of the external air, the external air will have a tendency to rush into such vessel or enclosed space with a force proportionate to the excess of the pressure of such external air over that of the air within; and if any communication be opened between the interior of such vessel or enclosed space, and the external air, the latter will rush in until an equilibrium be established between the pressures within and without. It is evident that the force thus obtained by diminishing the pressure of air within a vessel may be applied to any mechanical purpose.

It is by such means that water is raised in an ordinary pump. A portion of the air contained between the piston of the pump and the surface of the water below, is withdrawn by the action of the piston, and the pressure of the air remaining under the piston is thereby diminished. The superior pressure of the atmosphere upon the external surface of the water in the well then forces up a column of water in the pump-barrel, and this is continued as the air is more and more rarefied by the action of the piston. By whatever means, therefore, the air can be wholly or partially withdrawn from any space, a mechanical power will be thereby developed, proportional in its amount and efficacy to the quantity of air so withdrawn. If, however, such air be withdrawn by any mechanical process, such as by a syringe, by a common pump, or by an air-pump, the quantity of force expended in withdrawing it is always equivalent to the amount of mechanical power obtained by the vacuum or partial vacuum so produced. Indeed the power expended is greater than the power so obtained, inasmuch as the friction, leakage, &c. of the exhausting apparatus must be allowed for.

(27.)There are, however, various other means by which air may be partially expelled from a vessel besides the direct application of mechanical force. Thus if heat be applied to[Pg044]the vessel, the air, as has been already explained, will acquire increased elasticity, and will rush from the vessel with a force proportionate to the excess of its elasticity above that of the external air, and this process may be continued by increasing the heat to which the vessel is exposed, until a very considerable portion of the air has been expelled. If the orifice by which the air has escaped be then closed, and the vessel be allowed to cool, the air within, by having its temperature reduced to that of the external air, will lose all the elasticity which it had gained from the heat, and will be in the same condition as if an equivalent quantity of air had been withdrawn by any mechanical agent. The external air, therefore, will have a tendency to rush in with a force corresponding to the difference of pressures.

The process of filling thermometers with mercury shows one use of producing a high degree of rarefaction by heat. To construct the instrument it is necessary to fill the bulb and a part of the tube with mercury; but the bore of the tube is so small that the mercury cannot be introduced by any ordinary means. It is therefore held over flame until heated to a high temperature. The air within it gradually increasing in pressure as its temperature is raised, is forced through the small bore of the tube, until the pressure of the air within becomes no more than equal to the pressure of the external atmosphere; this air being so rarefied that quantity in the bulb bears a very small proportion to its contents at common temperatures. The mouth of the tube is then plunged into mercury, and as the bulb cools, the air within it loses its elasticity, and the superior pressure upon the external surface forces the mercury into the tube. This continues until the air remaining within the bulb has been so contracted, that its pressure combined with the weight of the mercury, shall balance the atmospheric pressure. The tube is then reversed, and the air which remained rises in a bubble to the surface, and escapes.

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