calculation by which observations made under any other pressure may be reduced to this standard: the law being kept in mind, that the weight is always directly, and the volume inversely, proportionate to the pressure. Suppose, for instance, that, with the barometer at 29 inches, we had found the weight of 100 cubic inches of air to be 29.9 grains, and wished to know what the weight would be at standard pressure-by the rule of proportion we shall find, 29: 30: 29.9: 30.903. Or, suppose that we had measured 100 cubic inches of air, at 29 inches, and wanted to know what the volume would be at 30 inches, we shall have 30: 29: 100: 96.6.

§ 48. In measuring gases over the water, or mercurial baths (10), it is also necessary to take care that the liquid

(10) The mercurial bath is here represented, by which gases which are liable to be absorbed by water may be collected and transferred; it is constructed of iron: a is the shallow part of the bath, and b the well for filling the jars; c is a glass jar, serving the purpose

within and without the vessel may stand at the same level; and where this cannot be effected by plunging it into the deeper part of the bath, the difference of level must be measured, and the following simple correction applied. Suppose a quantity of air confined over mercury, the level of which stands higher by two inches within the jar than without, it must be obvious that the pressure of the atmosphere balances not only the elasticity of the included air, but the weight of the mercury within the jar; the elasticity of the air must therefore be less than that of the atmosphere at the time, by an amount of which the difference of the level is the measure; under the circumstances, two inches must be deducted from the height of the barometer, and the correction for pressure applied as before. Should the difference of level occur with water, it may be reduced to the corresponding difference of mercury, by dividing it by 13.5, the difference of the specific gravities of the two liquids (11).

§ 49. And now we may describe a beautiful and convenient process of Sir J. Leslie, for taking the specific gravity of such solid substances as are porous, like charcoal, or in powder like

of a reservoir. It may be filled with any gas, by first pressing it down into the cylinder, in which is an iron core, the interval between which and the exterior case is filled with mercury, and then allowing the gas to rise into the small bell-glass placed in the basin of mercury, at d. A jar, partially filled with gas, is represented at a.

(11) The rarefaction of the air in any vessel connected with the air-pump, may be measured by gauges, acting upon the principle of the barometer. In figure 4 (page 32), k represents a barometer-tube, opening under the receiver, f, and dipping at its lower end into an open cistern of mercury. As the air under the receiver becomes rarefied, the superior elasticity of the atmosphere raises up a column of mercury, which, upon the supposition of the total abstraction of air, would stand at the same height as the barometer. The difference between the two, measured upon a scale of inches, indicates the elasticity of the residual air. The siphon-gauge, at i, does not begin to act till the rarefaction has been carried on to a very great extent. The upper part of the siphon is filled with mercury by boiling, which is kept in its place by the pressure of the air; when this is diminished to such a degree as not to be able to support a column of mercury equal in height to the upright part of the tube, it begins to fall away from the top, and to rise in the parallel leg of the siphon, and, as the equality of these two columns would denote a perfect vacuum above each, the elasticity of the air in the receiver, with which the instrument is connected, is measured by their difference.

sand, and which is founded upon the properties of atmospheric air. The only precautions necessary to be taken, are to prevent the existence of cavities unconnected with the air, or which may be so small as to have the property of absorbing gases. The apparatus consists of a glass tube about 3 feet long, and open at both ends; one third of its length is about 4ths of an inch in diameter, and two-thirds do not exceed ths of an inch; the narrower and the wider parts are connected together by an extremely fine slit, which suffers air to pass but retains any sand or powder. The wider end or mouth of the apparatus is ground flat, and can be shut so as to be air-tight by a ground glass plate (12). The substance to be proved, suppose sand, is put into the wide part of the tube, which being held in a vertical position is to have its narrow extremity immersed in mercury till the metal rises to the division between the two. The lid is then to be fitted on air-tight. In this state it will be evident that there can be no air in the tube, except that mixed with the sand in the upper cavity. If the barometer stand at 30 inches and the tube be lifted perpendicularly till the mercury stand at 15 inches above its surface in the containing vessel, it is obvious that the air in the inside will be subjected to a pressure of only half an atmosphere, and of course will dilate and fill precisely twice the space it originally occupied, and half the quantity contained in the sand will be transferred into the narrow part of the tube. The space which it will here occupy will, of course, be exactly the same as twice the quantity under double pressure: or, in other words, we have measured in the narrow tube the bulk of the air originally contained in the powder.

Now let the sand be removed, and the experiment repeated

(12) abcde, represents the long glass tube, the wide part of which extends from a to b; f is the glass plate fitted, by grinding, to the mouth, a; a is the glass jar for holding the mercury.

with the larger cavity filled with air alone. It is obvious that the quantity being greater, it will when dilated to double the bulk under a pressure of 15 inches, occupy a larger space, which will be measured upon the tube; but the expanded air in the narrow tube will always occupy exactly the same space which the whole occupied at the ordinary atmospheric pressure; hence the difference between the two spaces will be equal to the bulk of the solid matter in the sand. Now by marking the number of grains of water held by the narrow tube on a graduated scale attached to it, we can find at once what is the weight of a quantity of water equal in bulk to the solid matter contained in the sand; by comparing this with the weight of the sand we shall have its true specific gravity.

§ 50. The atmosphere presses upon the surface of the earth, and upon the surfaces of all bodies which are plunged into it, with the same force as that by which it supports the mercury in the barometer; and a column of mercury, 30 inches in height, whose base is one square inch, would weigh about 15 lbs.; and would press upon the earth with the same force: every body, therefore, upon the surface of the earth, at the level of the sea, supports an average pressure of 15 lbs. upon every square inch of its surface. That we are not sensible of this pressure on our own persons, and on all surrounding objects, is owing to its equality in all directions. From the fluidity of the atmosphere, the perfect mobility of its particles,—any force is equally distributed throughout its mass, and its gravity not only presses downwards, but upwards, and laterally, and in every direction alike. If we destroy this equilibrium, as we may easily do by the air-pump, the pressure becomes immediately manifest; almost the first stroke of the pump fixes the receiver to the plate, and after the air has been exhausted to the utmost we may raise the weight of the pump itself without detaching it. It is for the purpose of enabling them to bear this enormous pressure that such glasses are made of a spherical form. If a glass of a cubic shape be exhausted, it is speedily crushed to atoms; or, if a bottle of a similar shape be hermetically sealed, when filled with air of the usual density, and inclosed under a receiver from which the air is pumped, the elasticity of the included air no longer being counteracted by the exterior air, it will burst outwards with equal violence.

It is curious to remark how frequently common experience

has anticipated science in the application, if not in the formal announcement, of scientific principles. A beautiful illustration of the expansive power of heat, and the pressure of the atmosphere, occurs in baking a fruit pie. The cook inverts a cup in the dish, from which a portion of the air is expelled by the heat of the oven; when allowed to cool, the remainder contracts, and a partial vacuum being formed, the pressure of the atmosphere forces the juice to rise in the cup, and thus prevents its coming in contact with the crust, by which it would be absorbed.

§ 51. The surface of a man of ordinary stature is about 15 square feet, so that the atmospheric pressure upon his body amounts to 14 tons; this he sustains with perfect convenience, because every cavity of his body is distended with aëriform matter of the same elastic force. He also moves about under this enormous load without being conscious of its existence, owing to the equality of its action: could we suppose the air to cease to press in an upward direction, while its downward weight continued, he would be bound to the spot on which his foot rested as effectually as the rooted oak. Something of this we may indeed feel from the fatigue we endure in walking upon a stiff clay. In such a soil the air is more or less pressed from under the feet, and just upon the same principle that the boy's plaything, called a sucker, raises the weight of a large stone, a strong effort is necessary to overcome this unequal pressure.

Those who ascend into the higher regions of the atmosphere, by climbing lofty mountains, or by balloons, often feel inconvenience in their ears, and other cavities of their bodies, from the included air not having time to adjust itself to the diminished pressure of the outward medium; and it is no wonder that persons of weak constitutions often feel considerable discomfort from sudden changes of the weather, when barometric oscillations may indicate a change of pressure upon their bodies of 18 or 20 hundred weight in the course of a few hours.

The eminent philosopher and traveller, Baron Humboldt, mentions that he has in his own person experienced a difference of pressure equal to 31 inches of mercury; having been upon the summit of Chimborazo, where the barometer stood at 13 inches 11 lines, and in a diving bell under a pressure of 45 inches. Such is the action and reaction of weight and elasticity upon the air, and the equilibrium of the two forces in the atmosphere.