CHAPTER I. ON THE PROPERTIES OF AIR.

...medium, and continuing to pump, it will be expanded according to whatever proportion is desired above the previous level, by observing the divisions marked along the tube. If one continues to pump until the mercury is nearly level on both sides, one will see clearly how many piston strokes are required to evacuate all the gross air Original: "l'air grossier." In 18th-century science, this referred to common atmospheric air, which was thought to be a mixture of various particles, as opposed to "pure" air..

FIG. 11.

806. If one has a bottle containing mercury up to height AB, and a tube EF open at both ends, one end of which is submerged in the mercury near the bottom, and if the surface of the tube and the neck of the bottle are tightly joined so that air cannot escape the bottle; when the air in the receiver The bell jar of the air pump where the vacuum is created. expands, the mercury is seen to rise in the tube by the force of the spring of the air Original: "ressort de l'air." This term refers to the elasticity or pressure of air, which was famously studied by Robert Boyle. trapped in the bottle. This air, seeking to expand as well, presses upon the surface of the mercury; since the mercury is no longer pressed down at the opening of the tube, it is forced to rise until both forces are in equilibrium.

Gunpowder has no effect in the pneumatic machine.

807. If one places gunpowder inside and ignites it through the receiver by means of a burning glass A magnifying lens used to concentrate sunlight to create enough heat for ignition.; instead of igniting with a detonation, it merely melts and bubbles. This is because while the saltpeter and sulfur are melted by the heat, the air that was enclosed within the grains rarefies Becomes thinner or less dense through expansion., escapes, and causes the bubbling. This proves clearly—as I endeavored to suggest in The French Bombardier Original: "Le Bombardier François." This was Bélidor's own famous treatise on ballistics and the use of mortars, published in 1731.—that ignited powder is nothing but a fire with the property of putting air into action and releasing its "spring" with great rapidity. It is only the air thus rarefied that produces all the effects attributed solely to the powder, though quite incorrectly, since the powder ceases to act as soon as the molecules of air are missing. Since air has more or less "spring" depending on how much it is rarefied, and as heat, cold, and humidity continually cause considerable changes in it, it is not surprising that the same powder produces so many inequalities in its effects, since it necessarily feels all the variations of the air. This is why experiments relating to artillery require a precision far above what is usually applied, since one cannot know the source of the resulting changes without observing those occurring in the air at the same time. Thus, properly understood, it can be said that the art of throwing bombs becomes an object not only of a Geometry beyond common principles, but also of a very delicate Physics.