56 APPLIED SCIENCE 



if the bar is of the same thickness throughout its length, it will 

 be exactly in the center. If the support is changed to any 

 other point, the bar will fall to the ground; or if a weight of 1 

 Ib. be fixed on one end, and a weight of 4 Ibs. on the other end, 

 then the center of gravity will be 1 ft. from the 4-lb. weight. 

 The center of gravity is also called the center of inertia or the 

 center of mass. It is the point in a body about which the mass 

 is evenly disposed and if pivoted at that point, the body ought to 

 be balanced. 



56. The Line of Direction. A perpendicular line drawn 

 from the center of gravity to the earth is called the line of direc- 

 tion. This imaginary line is of great importance in the con- 

 struction of buildings, chimneys, and other tall struc- 

 tures. By the use of the law of gravity and the " plumb 

 line," the mason, bricklayer, or machinist can test a 

 wall or other kind of structure as it is being built to 

 see that it is perpendicular and perfectly straight. 



57. Mercury Plumb Bobs. Mercury plumb bobs 

 (Fig. 36) are usually made of hollow steel rods filled 

 with mercury or quicksilver. Consequently they are 

 unusually heavy in proportion to their size, and their 

 centers of gravity are low. Their comparatively small 

 diameters also allow them to be used close to corners 

 FIG 36 anc ^ wa ^ s J ^ ne v are not easily affected by draughts of 

 Mercury a ir; and they can be packed in a small space. As a re- 

 Bob, suit, they may be used to advantage almost anywhere. 



58. Acceleration Due to Gravity. If a body falls freely 

 in vacuum, that is, without resistance from the air, its ve- 

 locity will not be constant throughout the entire fall, but will 



