Page:Reflections on the Motive Power of Heat.djvu/259

parallel branches. Making the tube very long, the index would have a larger range for the same changes of pressure, and the results produced could then be measured by a slight variation in density in the air of the receiver.

Comparison of the Rapidity with which the Air cools in the Receiver and in the Tube.

Let us suppose, what I believe to be very near the truth, that the heat absorbed is proportional to the surface of the bodies in contact. From this we can infer without difficulty, that the rapidity of the cooling of the air in two cylindrical tubes would be inversely as their diameters.

If the receiver is considered as a tube of two decimetres in diameter, and the manometer as a tube of one millimetre diameter, the rapidity of the cooling of the air would be in the ratio, very nearly, of 1 to 200.

Extent of the Movement of the Index.

Suppose the tube turned up on itself five times and having a total length of 1 metre; a variation of density equal to ${\tfrac {1}{10}}$ in the air will give a movement of 1 decimetre; a variation of heat of 1 degree supposed to be equivalent to a variation of density of ${\tfrac {1}{266}}$ will give ${\tfrac {1}{266}}$ of a metre, or about