Although "trifles light as air" has become a proverb, yet air is positively heavy. A hogshead of air weighs about ten ounces; this is heavier than the gas which is burned in the streets and shops, of which a hogshead would weigh only seven ounces; and very much heavier than hydrogen gas, with which balloons were formerly filled, a hogshead of hydrogen gas weighing only two-thirds of an ounce. A balloon filled with hydrogen, or even with coal gas, rises into the air, as oil or a cork rises up through water. The air being thus heavy, presses upon the earth; and by measuring the degree of pressure we can tell how much air there is. This is done by an instrument termed a barometer--a glass tube closed at one end, and which, having been filled with quicksilver, is turned upside down in a cup containing quicksilver also. The tube being shut at the top, the air does not press on the quicksilver inside, but presses upon that in the basin; the quicksilver in the tube, which tends naturally to fall down into the basin, is thus forced to remain up in the tube by the pressure of external air; and it rises so high that the pressure inside, of the quicksilver, and outside, of the air, is equal. If the pressure of the air diminishes, the quicksilver falls; if the pressure of the air increases, the quicksilver rises: and as all great changes of the air are connected with changes of the weather, the barometer is generally known and consulted as a sort of weather-glass. Every space of an inch square supports fifteen pounds weight of air; at the rate of ten ounces to a hogshead, the depth of the air would therefore be about five miles. But it is much deeper, for air is what is termed compressible--that is to say, it may by pressure be squeezed into a smaller bulk; and hence the air next the ground, being compressed by the portions above it, is much the heaviest portion. At three miles high a hogshead of air weighs only five ounces, and at eight miles high only two ounces; hence the limits of the air are much farther removed, and it is known to extend to at least forty miles.