ASTRONOMY, ETC., OF SEVENTEENTH CENT. 211 



of temperature, are counteracted, or compensated as it is termed. 

 Methods of compensating pendulums for changes of length, due to 

 expansion and contraction of their suspending-rods, were afterwards 

 devised. One of the forms best known is that represented in Fig. 97, 

 and called the Harrison's Gridiron Pendulum. The weight o' is sus- 

 pended by a steel rod, which, at its upper end, is supported by a pair 

 of brass rods ; these in turn rest upon steel ; and so on alternately ; 

 the lengths of the rods being so adjusted that the unequal expansibility 





FIG. 97. 



FTG. 98. 



of the two metals is balanced, and the centre of gravity o' of the pen- 

 dulum weight remains always precisely at the same distance from the 

 point of suspension o. Clocks had been made before the time of 

 Huyghens, but they were deficient in the necessary principle of an exact 

 regulating power. Before wheel clocks came into use, astronomers and 

 others had to rely upon the indications of such instruments as \.\\z clep- 

 sydra or water-clock, Fig. 98. The clock is now an essential part of 

 the fittings of the astronomical observatory. The positions of the stars 

 are now determined by their height on the meridian, and by the times 

 at which they pass it. 



142 



