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.
Mrs. B. It is all that is essential to a barometer. The tube and the cup, or a cistern of mercury, are fixed on a board, for the convenience of suspending it; the brass plate on the upper part of the board, is graduated into inches, and tenths of inches, for the purpose of ascertaining the height at which the mercury stands in the tube; and the small moveable metal plate, serves to show that height, with greater accuracy.
Emily. And at what height, will the weight of the atmosphere sustain the mercury?
Mrs. B. About 28 or 29 inches, as you will see by this barometer; but it depends upon the weight of the atmosphere, which varies much, in different states of the weather. The greater the pressure of the air on the mercury in the cup, the higher it will ascend in the tube. Now can you tell me whether the air is heavier, in wet, or in dry weather?