236 APPENDIX A. 



3 mm .70, quite an appreciable quantity. As to the 

 time required to move the mercury index, regard 

 being had to its mass, if we suppose it 1 centi- 

 metre long, and the variation of pressure T J^ of an 

 atmosphere, it would require about of a second 

 to make it pass over one decimetre. 



Use of the Instrument in Measuring the Varia- 

 tions of the Tensions of the Air under a Pneu- 

 matic Receiver. 



At each stroke of the piston which expands the 

 air under the pneumatic receiver when a vacuum 

 is to be created, a lowering of pressure is produced, 

 and undoubtedly a change of temperature. It can 

 be determined approximately, at least, by observing 

 the position of the manometer at the instant after 

 the dilatation has taken place, and again after a 

 time long enough for the temperature to have re- 

 turned to its original point, that of the surrounding 

 bodies. Comparison of the elastic force in the two 

 cases will lead to comparison of the temperatures. 



The temperature having returned to its original 

 point, we will give a second stroke of the piston 

 which will rarefy the air more than the former, 

 and thus we will make two observations of the 

 manometer, before and after the return to the 

 former temperature. And so on. 



