ASTRONOMY. 





Fig. I.- Mercurial Pendulum. 



Figures 1 and 2 illustrate contrivances for orer- 

 cuining the changes, produced by heat, in th 

 pt-ndula of clocks referred to at page 1001. 

 In the Gridiron Pendulum, this is effected by 

 employing rods of different metals, or the same 

 metals, o fixed, as that whilst one scries ex- 

 pands or contracts in one direction, the other 

 shall do so in an opposite one. In the Mercurial 

 Pendulum, a cistern containing mercury is em- 

 ployed, which, by Its changes of Irng.n, com- 

 pensate* for those of the rod of the pendulum ; 

 that being the object of each arrangement. 



Fig. S gires a sectional riew of the Micrometer- 

 Microseope, which has been already described 



fi(. S.- Micrometer-Microscope. 



