372 THE APPLICATIONS OF PHYSICAL FORCES. [BOOKIV. 



the pivots t, t, and so raise it that the centre of oscillation tends to 

 rise at the same time that the expansion of the suspending rod tends 

 to lower it. The compensation is in this manner partly effected, and 

 it only remains to complete it, as before, by trial. 



Graham's compensation pendulum is that generally employed in 

 England, and is formed of a steel rod, which supports at its lower end 



FIG. 255. Graliaiii's compensation pendulum. 



FIG. 250. Ellicott s compensation pendulum. 



a stirrup containing one or sometimes two glass cylinders, partly filled 

 with mercury. When the expansion gives rise to an elongation of the 

 suspending rod, and lowers the centre of oscillation in consequence, 

 it is at the same time raised by the elevation of the level of the 

 mercury in the tubes, arising from the same increase of temperature. 

 The amount of mercury required is easily calculated from a 'table 



