564 



collision with material objects. In fact we find that in sheltered 

 situations, such for instance as one or two inches above a wall op- 

 posite to the wind, the thermometer indicates a higher temperature 

 than it does when exposed to the blast. The question, which is one 

 of great interest for meteorological science, has hitherto been only 

 partially discussed by us, and for its complete solution will require a 

 careful estimate of the temperature of the earth's surface, of the 

 effects of radiation, &c., and also a knowledge of the causes of gusts 

 in different winds. 



XXII. " On the Thermal Effects of Longitudinal Compression 

 of Solids." By J. P. JOULE, Esq., F.R.S. ; and " On the 

 Alterations of Temperature accompanying Changes of 

 Pressure in Fluids." By Prof. W. THOMSON, F.R.S. 

 Received June 18, 1857. 



In the further prosecution of the experiments of which an out- 

 line was given in the ' Proceedings' for January 29, 1857, the author 

 has verified the theory of Professor Thomson, as applied to the 

 thermal effects of laying weights on and taking them off metallic 

 pillars and cylinders of vulcanized india-rubber. Heat is evolved 

 by compression, and absorbed on removing the compressing force 

 in every substance yet experimented on. In the case of metals, the 

 results agree very closely with the formula in which e, the longi- 

 tudinal expansion by heat under pressure, is considered the same 

 as the expansion without pressure. It was observed, however, that 

 all the experimental results were a little in excess of the theoretical, 

 and it became therefore important to inquire whether the force 

 of elasticity in metals is impaired by heat. In the first arrange- 

 ments for this purpose, the actual expansion of the bars employed 

 in the experiments was ascertained by a micrometric apparatus, 1st, 

 when there was no tensile force, and 2nd, when a weight of 700 Ibs. 

 was hung to the extremity of the quarter-inch rods. The results, 

 reliable to less than one-hundredth of their whole value, did not 

 exhibit any notable effect of tensile force on the coefficient of ex- 

 pansion by heat. An experiment susceptible of greater delicacy 



