been previously applied to work the pistons; so that there is really no gain of power. The advantage is, that, by interposing the air, we get an agent which acts by continuous pressure, and not by intermitting strokes. So in the airgun: by condensing air within the breech of the gun, we procure a force, which, on being released, will act instantaneously, and with a power equal to all the separate effects made in the process of condensing it. If we attempted, by mere manual force, to project a bullet, we should produce but little effect. Condensed air serves as a magazine, in which we can treasure up and combine a great number of these separate exertions of our strength, and cause them to act simultaneously.

It may give us some impression of the value of science, as connected with the arts, if we consider, that scarcely one of the important powers which we have just considered was employed by the Greeks and Romans, in the days of their greatest civilization and renown. They had neither waterwheels,* windmills, nor airpumps. To procure bread for his family, a Greek had no flour-mills, except such as were moved by hand or by animals. Neither had he sawmills, to supply boards and lumber for edifices; nor fire-engines, to rescue cities and habitations from the devouring element; nor hydrostatic presses, to concentrate, within a small compass, immense pressure; nor metallic pipes, to convey water over hills and valleys, without the expense of arches and mason work; nor, in fine, the airpump, to withdraw substances from the contact of the atmosphere, so as to observe the effect on combustion, sounds, respiration, and vegetable life.

But, while we acknowledge the vast benefits which

*This remark requires some qualification. Beckmann says: "The first certain information we have of the invention of watermills is not older than the time of Julius Cæsar. Cattlemills continued in such general use, that, near three centuries afterwards, there were more than three hundred at Rome; and, A. D. 398, some public enactments were made, which show that, even then, watermills were considered a new establishment."

have been conferred by these inventions, in modern times, we must remember, that even yet the principles on which they depend are by no means generally understood. Multitudes, even of those who are called to deal continually with those principles, have no proper conception of them, and commit many blunders and waste a great deal of money, which a little knowledge would have enabled them to save. The two following instances will be sufficient illustration of this fact: “ A respectable gentleman, of landed property, in one of the middle counties of Scotland, applied to a friend, a lecturer on chemistry and natural philosophy, in order to obtain his advice respecting a pump-well, which he had lately constructed at considerable expense. He told him, that, notwithstanding every exertion, he could not obtain a drop of water from the spout, although he was quite sure there was plenty of water in the well, and although he had plastered it all around, and blocked up every crevice. When his friend inspected the pump, he suspected that the upper part of the well was airtight, and, consequently, that the atmospheric pressure could not act on the surface of the water in the well. He immediately ordered a hole to be bored, adjacent to the pump, when the air rushed in, with considerable force; and, on pumping, the water flowed copiously from the spout. The gentleman was both overjoyed and astonished; but it is somewhat astonishing, that neither he, nor his neighbors, nor any of the workmen who had been employed in its construction, should have been able to point out the cause of the defect; but, on the other hand, should have taken the very opposite means for remedying it, namely, by plastering up every crevice, so as to produce a kind of vacuum within the well."*

* Dick, on the Improvement of Society. We once knew a man to carry a series of aqueduct pipes to the bottom of a well sixty feet deep, and thence over the top down into a neighboring valley, below the level of the bottom of the well, for the purpose of supplying water to a small manufactory, as a syphon. If he had known that water rises in the syphon, as in the pump, in consequence of

7

S. A.

After the diving-bell was invented, it was considered desirable to devise some means of remaining, for any length of time, under water, and rising at pleasure, without assistance. "Some years ago, an ingenious individual proposed a project, by which this end was to be accomplished. It consisted in sinking the hull of a ship, made quite watertight, with the decks and sides strongly supported by shores, with the only entry secured by a stout trapdoor, in such a manner, that, by disengaging from within, the weights employed to sink it, it might rise of itself to the surface. To render the trial more satisfactory and the result more striking, the projector himself made the first essay. It was agreed, that he should sink in twenty fathoms water, and rise again, without assistance, at the expiration of twentyfour hours. Accordingly, making all secure, fastening down his trapdoor, and provided with all necessaries, as well as with the means of making signals, to indicate his situation, this unhappy victim of his own ingenuity entered, and was sunk. No signal was made, and the time appointed elapsed. An immense concourse of people had assembled to witness his rising, but in vain for the vessel was never seen more. The pressure of the water at so great a depth had no doubt been completely under-estimated; and the sides of the vessel being at once crushed in, the unfortunate projector perished, before he could even make the signal concerted, to indicate his distress."*

the pressure of the atmosphere, and that this pressure will force it up only a little more than thirty feet, he might have been spared the cost of the attempt and the mortification of its failure.

* Sir J. Herschel, on the Study of Natural Philosophy.

CHAPTER VI

MECHANICAL AGENTS.- -(INANIMATE FORCES CONTINUED.)

III. Heat.-HAVING thus explained how gravity and elasticity are employed, as moving forces in the arts, we come, in the third place, to consider Heat.

Its use as a mechanical agent depends upon the power which it has of expanding bodies. If a bar of metal be accurately measured, and then raised to a red heat, it will be found longer than before; and still longer, if it be raised to a white heat. It is for this reason that the mechanic heats the iron tire of a wagon or coach wheel, and the iron hoops of a cask, before putting them on; and also the rivets which are used in binding together the iron plates of which boilers are made. Being, by this means, expanded, they are easily fitted to their places, while the contraction which follows binds the adjacent parts together, and holds them firmly.

A striking instance of the use, as a mechanical agent, to which this expansive power of metals may be applied, occurred some years since, in Paris. The weight of the roof of the abbey of St. Martin was forcing the walls asunder, and the following method was taken to restore them to their perpendicular position. Holes were made, at opposite points, in several parts of the wall, through which strong iron bars were introduced, so as to extend across the building, their ends projecting outside the walls. Large nuts were placed upon their ends, and screwed up, so as to press upon the walls. Every alternate bar was then heated, by powerful lamps, so that its length was increased by expansion, and the nuts, before in close contact with the walls, retired to some distance from them: the nuts were then screwed up to the walls, and the bars cooled. The pro

cess of cooling restored the length of the bars to what it had been before the heat was applied, and the nuts were drawn together by an immense force. The same process being repeated with the intermediate bars, and this being continued, the walls of the building were gradually restored to their perpendicular position.

The expansion of bodies by heat affords us the means of constructing instruments to measure different degrees of temperature. The principal of these are, the common thermometer, and the pyrometer of Wedgewood. The thermometer consists of a small tube, terminating at the bottom in a bulb, containing mercury or spirits of wine. The air having been previously expelled by heat, the tube is closed at the top, to prevent its return, and a vacuum being thus produced, above the fluid, it is free to expand when heat is applied. This expansion is indicated by a rise of the fluid; whereas the contraction produced by cold causes the fluid to descend. A scale, attached to the tube, and divided into degrees, measures these variations, and thus indicates the temperature. In dividing this scale, it is usual, in the first place, to fix the two points, at which the mercury stands, when the temperature is sufficiently low to freeze water, and sufficiently high to boil it. These are obtained, by immersing the instrument with the scale, first into melting ice, and afterwards into boiling water. The intermediate space is then divided into a certain number of degrees, the number being different in different kinds of thermometers. If the bore of the tube be throughout of the same size, these spaces or degrees will be equal in length; otherwise, they will be unequal. It is evident, that that portion of the tube to which they correspond must always be of the same capacity.

The pyrometer is an instrument used for measuring higher degress of temperature than are indicated by ordinary thermometers. It depends upon a property of pure clay, which forms an exception to the general