489 



between them curved, as in a a', b b', they will rush together. If one be wet 

 and another dry, as C C', they will repel one another at a certain distance; but, 

 as soon as they are brought very near, they will rush together, as in the former 

 cases. 



NOTE 173, p. 149. Latent heat. There is a certain quantity of heat in all 

 bodies, which cannot be detected by the thermometer, but which may become 

 sensible by compression. 



NOTE 174, p. 153. Reflected waves. A series of waves of light, sound, or 

 water, diverge in all directions from their origin I, fig. 43, as from a centre. 



Fig. 43. 



When they meet with an obstacle S S, they strike against it, and are reflected 

 or turned back by it in the same form, as if they had proceeded from the 

 centre C, at an equal distance on the other side of the surface S S. 



NOTE 175, p. 153. Elliptical shell. If fig. 6 be a section of an elliptical shell, 

 then all sounds coming from the focus S to different points on the surface, as 

 m, are reflected back to F, because the angle T m S is equal to t m F. In a 

 spherical hollow shell, a sound diverging from the centre is reflected back to 

 the centre again. 



NOTE. 176, p. 158. Fig. 44 represents musical strings in vibration; the 

 Fig. 44. 



a 



