_Mrs. B._ That weight, you see, is almost two grains. The dimensions of this bottle, are six cubic inches. Six cubic inches of air, therefore, at the temperature of this room, weighs nearly 2 grains. _Caroline._ Why do you observe the temperature of the room, in estimating the weight of the air? _Mrs. B._ Because heat rarefies air, and renders it lighter; therefore the warmer the air is, which you weigh, the lighter it will be. If you should now be desirous of knowing the specific gravity of this air, we need only fill the same bottle, with water, and thus obtain the weight of an equal quantity of water--which you see is 1515 grs.; now by comparing the weight of water, to that of air, we find it to be in the proportion of about 800 to 1. As you are acquainted with decimal arithmetic, you will understand what I mean, when I tell you, that water being called 1000, the specific gravity of air, will be 1.2. I will show you another instance, of the weight of the atmosphere, which I think will please you: you know what a barometer is? _Caroline._ It is an instrument which indicates the state of the weather, by means of a tube of quicksilver; but how, I cannot exactly say. _Mrs. B._ It is by showing the weight of the atmosphere, which has great influence on the weather. The barometer, is an instrument extremely simple in its construction. In order that you may understand it, I will show you how it is made. I first fill with mercury, a glass tube A B, (fig. 3, plate 14.) about three feet in length, and open only at one end; then stopping the open end, with my finger, I immerse it in a cup C, containing a little mercury. _Emily._ Part of the mercury which was in the tube, I observe, runs down into the cup; but why does not the whole of it subside, for it is contrary to the law of the equilibrium of fluids, that the mercury in the tube, should not descend to a level with that in the cup? _Mrs. B._ The mercury that has fallen from the tube, into the cup, has left a vacant space in the upper part of the tube, to which the air cannot gain access; this space is therefore a perfect vacuum; the mercury in the tube, is relieved from the pressure of the atmosphere, whilst that in the cup, remains exposed to it. _Caroline._ Oh, now I understand it; the pressure of the air on the mercury in the cup, forces it to rise in the tube, where there is not any air to counteract the external pressure. _Emily._ Or rather supports the mercury in the tube, and prevents it from falling. _Mrs. B._ That comes to the same thing; for the power that can support mercury in a vacuum, would also make it ascend, when it met with a vacuum. Thus you see, that the equilibrium of the mercury is destroyed, only to preserve the general equilibrium of fluids. _Caroline._ But this simple apparatus is, in appearance, very unlike a barometer.