26 UNIT OF MECHANICAL FORCE. PARTI. 



the mercury, and its surface is concave, while that of 

 the mercury is convex. The elevation or depression of 

 the same liquid in different tubes of the same matter is 

 in the inverse ratio of their internal diameters, and 

 altogether independent of their thickness; whence it 

 follows that molecular action is insensible at sensible 

 distances, for when tubes of the same bore are wetted 

 throughout their whole extent with water, mercury 

 will rise to the same height in all of them whatever be 

 their thickness or density, the film of water being 

 sufficient to intercept the molecular action, and to 

 supply the place of a tube by its own capillary attrac- 

 tion. The action of this force is daily seen in the 

 absorption of water by sponges, sugar, salt and other 

 porous bodies, and it is a most important agent in the 

 circulation of fluids in animals and vegetables.' 



Every atom of matter is subject to the force of gravi- 

 tation, but each substance has its own peculiar weight 

 of specific gravity, that is to say, the same bulk of dif- 

 ferent substances contains different quantities of matter. 

 Since nothing is known of absolute weight it is neces- 

 sary to have some standard of comparison, and for that 

 purpose pure water at the temperature 39 Fahr. (that 

 of its maximum density) is chosen for solids and liquids ; 

 while for gases and vapours atmospheric air at the tem- 

 perature of sixty degrees of Fahrenheit's thermometer, 

 and a barometric pressure of thirty inches, is assumed 

 as the unit of specific gravity. 



The foot-pound, which is the unit of mechanical force 

 as established by Mr. Joule, is the force that would 

 raise one pound of matter to the height of one foot ; 

 or it is the impetus or force generated by a body of one 

 pound weight falling by its gravitation through the 

 height of one foot. Now impetus or vis viva is equal to 

 the mass of a body multiplied by the square of the 

 velocity with which it is moving : it is the true measure 

 of work or mechanical labour. For if a weight be raised 



