120 



THEORY OF HEAT. 



[CHAP. II. 



other molecules, that is to say, during each infinitely small instant 

 its temperature would increase or decrease by a finite quantity, 

 which is contrary to experience. 



151. We proceed to apply this remark to a molecule situated 

 at the external surface of the solid. 



Fig. 6. 



a 



Through a point a (see fig. 6), taken on the plane of x and y, 

 draw two planes perpendicular, one to the axis of x the other to 

 the axis of y. Through a point b of the same plane, infinitely 

 near to a, draw two other planes parallel to the two preceding 

 planes ; the ordinates z, raised at the points a, b, c, d, up to the 

 external surface of the solid, will mark on this surface four points 

 a , b , c , d , and will be the edges of a truncated prism, whose base 

 is the rectangle abed. If through the point a which denotes the 

 least elevated of the four points a , b , c, d r , a plane be drawn 

 parallel to that of x and y, it will cut off from the truncated prism 

 a molecule, one of whose faces, that is to say ab c d , coincides 

 with the surface of the solid. The values of the four ordinates 

 a a , cc, dd } bb are the following : 



aa f z, 



77 i j 



bb = z -f- -y- dx -f- -j- d&amp;gt;/. 

 dx dy J 



