234 APPENDIX A. 



when the index is on 1. Whether the air be ex- 

 pelled or admitted the pressures would be instead 

 of p and^/, q and q r . 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 tempera- 

 ture and note the variations. 



If the straight part of the tube were perfectly 

 calibrated, the volumes, and consequently the pres- 

 sures, 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 at- 

 mospheric 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 re- 

 ceiver 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 mors 



