The two following examples will be sufficient to explain these two tables, and show their agreement. Ex. I.-In 6835 pounds 6 ounces 9 pennyweights 6 grains Troy, Qu. How much Avoirdupois weight? (See page 186.) Ex. II.—In 5624 pounds 10 ounces 12 drams Avoirdupois, Qu. How much Troy weight? (See page 187.) 189 LECTURE VI. of Pneumatics. THIS science treats of the nature, weight, pressure, and spring, of the air, and the effects arising therefrom. perties of The air is that thin transparent fluid body The proin which we live and breathe. It encompasses the whole earth to a considerable height; and, together with the clouds and vapours that float therein, is called the atmosphere. The air is justly reckoned among the number of fluids, because it has all the properties by which a fluid is distinguished. For, it yields to the least partial force impressed, its parts are easily moved among one another, it presses according to its perpendicular height, and its pressure is every way equal. That the air is a fluid, consisting of such particles as have no cohesion between them, but easily glide over one another, and yield to the slightest impression, appears from that ease and freedom with which animals breathe in it, and move through it without any difficulty or sensible resistance. But it differs from all other fluids in the four following particulars: 1. It can be compressed into a much less space than what it naturally possesses, which no other fluid can: 2. It cannot be congealed or fixed, as other fluids may : 3. It is of a different density in every part, upward from the earth's surface, decreasing in its weight, bulk for bulk, the higher it rises; and therefore must also decrease in density: 4. It is of an elastic or springy nature, and the force of its spring is equal to its weight.* That air is a body, is evident from its excluding all other bodies out of the space it possesses for, if a glass jar be plunged with its mouth downward into a vessel of water, there will but very little water get into the jar, because the air of which it is full keeps the water out. As air is a body, it must needs have gravity or weight and that it is weighty, is demonstrated by experiment. For, let the air be taken out of a vessel by means of the air-pump, then, having weighed the vessel, let in the air again, and upon weighing it when refilled with air, it will be found considerably heavier.Thus, a bottle that holds a wine-quart, being emptied of air and weighed, is found to be about 16 grains lighter than when the air is let into it again: this shows that a quart of air weighs 16 grains. But a quart of water weighs 14,621 grains; this divided by 16, quotes 914 in round numbers; which shows, that water is 914 times as heavy as air near the surface of the earth. As the air rises above the earth's surface, it grows rarer, and consequently lighter, bulk for * As the compressibility of water and other fluids is now completely ascertained, air will differ from these only in one circumstance, ⚫ that it cannot be congealed or fixed.' The water of the ocean, and even smaller quantities of liquid, must have different densities, (however small the difference) on account of the weight of the superincumbent fluid; and every compressible fluid must be elastic. The elasticity of water is completely proved by the reflection of stones impinging upon its surface.-E. ED. Most of the elastic fluids, by the modern chemists termed gases, possess the above properties in common with atmospheric air; though each has besides its own distinguishing properties. A. ED. |