The fall of the mercury in the long arm of the tube would cause the weight F to be pressed upwards. This would release the string to which the weight W is attached; it would, therefore, fall, and turn the hand down to Rain or Much Rain.
700. Why does the rise of the barometer denote the approach of fine weather?
Because the external air becoming dense, and free from highly elastic vapours, presses with increased force upon the mercury upon which the weight F floats; that weight, therefore, sinks in the short tube as the mercury rises in the long one, and in sinking turns the hand to Change, Fair, &c.
"He caused an east wind to blow in the heaven; and by his power he brought in the south wind."—Psalm lxxviii.
701. Why does the barometer enable us to calculate the height of mountains?
Because, as the barometer is carried up a mountain, there is a less depth of atmosphere above to press upon the mercury; it therefore falls, and by comparing various observations, it has been found practicable to calculate the height of mountains by the fall of the mercury in a barometer.
702. To what extent of variation is the weight of the atmosphere liable?