74 



THE PENDULUM. 



ity of the spring. Bat the weights have a tendency toward 

 accelerated motion (falling bodies), while the spring would 

 give an example of diminishing motion. Either defect 

 .would be fatal in a time-piece. Hence the properties of 

 the pendulum set forth in the first and third laws are 

 used to regulate this motion and make it available for the 

 desired end. If the clock gains time, the pendulum is 

 lengthened by lowering the bob; if it loses time, the pen- 

 dulum is shortened by raising the bob. 



149. Compensation Pendulums. The expan- 

 sion of metals by heat is a familiar fact. Hence the ten- 

 Idency of a clock to lose time in summer and 

 to gain time in winter. One plan for coun- 

 teracting this tendency is by the use of the 

 " gridiron " pendulum which is made of two 

 substances in such a manner that the down- 

 ward expansion of one will be exactly com- 

 pensated by the upward expansion of the 

 other. In the figure, the heavy single lines 

 represent steel rods, the effect of whose ex- 

 pansion will be to lower the bob. The light 

 double lines represent brass rods, the effect of 

 whose expansion will be to raise the bob. The 

 steel rod to* which the bob is directly attached 

 passes easily through holes in the two hori- 

 zontal bars which carry the brass uprights. 



FIG. 35- 



As brass expands more than steel, for a given increase of 

 temperature, it will be seen that these two expansions may 

 be made to neutralize one another. 



