on the Subject of Heat. 211 



is too complicated to be represented in this place ; * 

 still it will not be difficult for the reader, with the 

 help of the accompanying figure (see Plate V.), to 

 form a conception of the principal experiments and 

 their results. 



Let A be the vertical section of a brass rod which is 

 an inch in diameter and is fastened in an upright posi- 

 tion on a stout block, B ; it is provided at its upper 

 end with a massive hemisphere of the same metal, three 

 and a half inches in diameter. C is a similar rod, like- 

 wise vertical, to the lower end of which is fastened a 

 similar hemisphere. Both hemispheres must fit each 

 other in such a way that both the rods stand in a per- 

 fectly straight vertical line. 



D is the vertical section of a globular metallic vessel 

 twelve inches in diameter, which is provided with a 

 cylindrical neck three inches long and three and three- 

 quarters inches in diameter. The rod A goes through 

 a hole in the bottom of the vessel, is soldered into the 

 vessel, and serves as a support to keep it in its proper 

 position. 



The centre of the ball, made up of the two hemi- 

 spheres which lie the one upon the other, is in the 

 centre of the globular vessel, so that, if the vessel is filled 

 with water, the water covers the ball as well as a part 

 of each of the brass rods. 



If now the hemispheres be pressed strongly together, 

 and at the same time the rod C be turned, by some 

 means or other, about its axis, a very considerable 

 quantity of heat is generated by means of the friction 

 which takes place between the flat surfaces of the two 

 hemispheres. 



* See Vol. I., Plate to p. 493. 



