178 



caused by currents of air of different velocities thus produced, he 

 was enabled to arrive at a measure of the velocities in tubes placed 

 in a still atmosphere, as described in his former paper. 



The author in that paper pointed out a correspondence between 

 the variations of force in the upward currents of atmospheric air in 

 the tubes and variations in the humidity of the atmosphere, and 

 expressed his belief that the variations were attributable in great 

 measure to the varying hygrometric conditions of the atmosphere. 



In further proof of this position, he has appended two tables, 

 showing that both natural and artificial increase of atmospheric 

 humidity are accompanied by increase in the velocity of the rotations, 

 and that in each case increase of humidity is attended by increase of 

 velocity, independent of temperature. 



III. " On the Thermal Effects of Fluids in Motion." By J. P. 

 JOULE, Esq., F.R.S., and Professor W. THOMSON, F.R.S. 

 Received May 23, 1856. 



On the Temperature of Solids exposed to Currents of Air. 



In examining the thermal effects experienced by air rushing through 

 narrow passages, we have found, in various parts of the stream, very 

 decided indications of a lowering of temperature (see Phil. Trans. 

 June 1853), but never nearly so great as theoretical considerations 

 at first led us to expect, in air forced by its own pressure into so 

 rapid motion as it was in our experiments. The theoretical investi- 

 gation is simply as follows : Let P and V denote the pressure and 

 the volume of a pound of the air moving very slowly up a wide pipe 

 towards the narrow passage. Let p and v denote the pressure and 

 the volume per pound in any part of the narrow passage, where 

 the velocity is q. Let also e E denote the difference of intrinsic 

 energies of the air per pound in the two situations, Then the equa- 

 tion of mechanical effect is 



since the first member is the mechanical value of the motion, per 



