Page:Reflections on the Motive Power of Heat.djvu/258

234 when the index is on 1. Whether the air be expelled or admitted the pressures would be instead of p and p', q and q'. But there would follow p : p' : : v' : v and q : q' : : v' : v ;  then p : p' : : q : q'.

We should moreover work at a uniform temperature and note the variations.

If the straight part of the tube were perfectly calibrated, the volumes, and consequently the pressures, would form a geometrical progression, when the figures of the scale would be found to be in arithmetical progression, and a table of logarithms would enable one to be found from the other.

In order to increase as required the mass of air enclosed in the tube the instrument must be placed on its side or flat, in the air-pump receivers. The mercury index would be placed in the lateral part of the enlargement of the tube, and the atmospheric air would enter. The instrument might also be heated in this position.

Care must be taken to admit only very dry air, which could be obtained by placing under the receiver calcium chloride or any other substance which absorbs moisture greedily.

Instead of bending the tube into a spiral, it might be bent in the middle in the form of a U, or it might be better to form three, four or more