ON EARTH TREMORS. 



293 



ending in a brass cap 2^ inches high, so that the total height of the 

 instrnment above the top of the foundation is only 19§ inches. 



A section of the instrnment by an east and west plane is given 



m 



Fig. 3. 



fig. 3,' and shows the manner in which the pendulum-mirror, m, is sus- 

 pended. Two ^-inch square brass bars, h and k, are firmly screwed 

 together (h being on the east side), and carry between them two pulley 



' In figs. 2 and 3 the same letters denote the same parts of the instniment. 



