74 



SCIENTIFIC RECREATIONS. 



\ 



and Doctor Joule, of Manchester, separately, and by different means, arrived 

 at the conclusion that a certain definite amount of mechanical work corre- 

 sponds to a certain definite amount of Heat, and vice versa. Thus was a 

 great support afforded to the Dynamic theory. This fact Doctor Joule 

 communicated to the PJtilosophical Magazine in 1843, an d the conclusions 

 he came to were 



1. "That the quantity of heat produced by the friction of bodies, whether 



solid or liquid, is always in proportion to the force expended ; 



2. " That the quantity of heat capable of increasing the temperature of 



a pound of water (weighed in vacua and taken at between 55 and 

 60 Fahr.) by i Fahn, requires for its evolution the expen- 



Fig. 76. Melting a piece of tin on a card. 



diture of a mechanical force represented by the fall of 772lbs. through the 

 space of one foot" 



This is the " mechanical equivalent of heat." The first paper written 

 by Mr. Joule demonstrated that the temperature of water rises when forced 

 through narrow tubes ; and to heat it one degree, the force of 770 foot 

 pounds was necessary, which means that the I Ib. of water falling 770 feet, 

 got hotter by one degree when it reached the earth. He subsequently 

 arrived at the more exact conclusions quoted above. 



So heat is now known to be a series of vibrations, or vibratory motions, 

 as sound vibrations, which we cannot hear nor see, but the effects of which 

 are known to us as light and heat. 



