APPENDIX A. 235 



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 rapid- 

 ity 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 T V in the air will give a move- 

 ment of 1 decimetre; a variation of heat of 1 de- 

 gree supposed to be equivalent to a variation of 

 density of ^ will give ^ of a metre, or about 



