CHAP, iv.] APPLICATIONS OF THE LAWS OF HEAT. 369 



up to Forbes and Tyndall, have obtained no results. But Melloni in 

 1846, and since then Piazzi Smith, Lord Rosse, and Marie Davy have 

 perceived a sensible heating. 



FIG. 252. A burning-glass with polyzonal lenses. 



II. COMPENSATED PENDULUMS. 



We have seen in the chapter on the applications of gravitation 

 that treats of the pendulum, that one of the essential conditions of 

 its employment in clocks is the constancy of the length of the rod, 

 or rather of the distance between the point of suspension and the 

 centre of oscillation. 



Now this constancy supposes that the temperature at which the 

 clock has been regulated with this pendulum itself remains constant. 

 For if it rises, the material of which the pendulum is formed dilates, 

 the pendulum is lengthened, and its oscillations are slower. If, 

 on the contrary, the temperature falls, the material" contracts, the 

 pendulum is shortened, and its oscillations are quicker. Whence it 



B B 



