Page:On the economy of machinery and manufactures - Babbage - 1846.djvu/63

Rh portable instrument than the barometer. It is well known that the barometer indicates the weight of a column of the atmosphere above it, whose base is equal to the bore of the tube. It is also known that the density of the air adjacent to the instrument will depend both on the weight of air above it, and on the heat of the air at that place. If, therefore, we can measure the density of the air, and its temperature, the height of a column of mercury which it would support in the barometer can be found by calculation. Now the thermometer gives information respecting the temperature of the air immediately; and its density might be ascertained by means of a watch and a small instrument, in which the number of turns made by a vane moved by a constant force, should be registered. The less dense the air in which the vane revolves, the greater will be the number of its revolutions in a given time: and tables could be formed from experiments in partially exhausted vessels, aided by calculation, from which, if the temperature of the air, and the number of revolutions of the vane are given, the corresponding height of the barometer might be found.