Chapter I. On the Properties of Air.

Part I. Vol. II. B

...closed by a key y, this key is bored through, like those of ordinary fountains, and at an equal distance from the two ends of the hole. On the surface of the key, on one side only, is a groove or slit A, half a line A pre-metric unit of measurement; in 18th-century France, a "ligne" was 1/12 of an inch, or about 2.25 millimeters. in width by one line in depth.

The plate AC is pierced in the middle by a hole X, soldered to the orifice of a small pipe, the other end of which connects to the faucet; a piece of wet leather is applied to the plate, upon which one places a glass bell Z, called a receiver, the effect of which is as follows.

Fig. 4.

800. Assuming that the piston Q immediately touches the head of the syringe, one turns the key y to leave the communication between the receiver and the syringe free; then, the coarse air original: "l'air grossier"; referring to common atmospheric air at standard pressure. that was in the body of the syringe having been driven out, the air in the receiver, finding room to expand, spreads into the body of the syringe; so that if one assumes for a moment that the capacity of the syringe is equal to that of the receiver, the air, now occupying double the space, is twice as expanded—or, if you prefer, half as condensed—as the air we breathe, since no other air can have entered. When the piston is at the bottom, the key y is turned in another direction to interrupt the communication between the receiver and the syringe; then if one removes one's foot from the stirrup S, the spring original: "ressort"; the elastic force or pressure of the air. of the external air pushes the piston from bottom to top, making it rise back up to half the distance it traveled while descending—that is, until the air in the syringe is reduced to the same degree of condensation as the air outside. If one then pushes the piston rod to make it rise toward the head of the syringe, the air in the body of the syringe will become more compressed than the air outside and will exit through the small slit A on the key y. If one turns the key again in another direction and makes the piston descend, the air that had remained in the receiver will expand yet again, and will have only a quarter of the spring it had in its natural state. Repeating the same maneuver several times, one will succeed in removing the greater part of the coarse air from the receiver; for one must not count on a perfect vacuum: all one can do is increase the expansion more and more by a greater number of piston strokes.

Fig. 5.

Method of knowing to what degree the air is expanded in the pneumatic machine.

801. To know, after a certain determined number of piston strokes, by how much the air remaining in the receiver is more expanded than that which was first enclosed there, one must pay attention that the expansion of the air enclosed in the receiver, whatever it may be, is always relative to the expansion of that which remains immediately after each...