A HISTORY OF SCIENCE 



presented to the Royal Society a second and some- 

 what larger telescope, which he had made; and this 

 type of instrument was little improved upon until the 

 introduction of the achromatic telescope, invented by 

 Chester Moor Hall in 1733. 



As is generally known, the element of accurate 

 measurements of time plays an important part in the 

 measurements of the movements of the heavenly bodies. 

 In fact, one was scarcely possible without the other, 

 and as it happened it was the same man, Huygens, who 

 perfected Kepler's telescope and invented the pendu- 

 lum clock. The general idea had been suggested by 

 Galileo ; or, better perhaps, the equal time occupied by 

 the successive oscillations of the pendulum had been 

 noted by him. He had not been able, however, to 

 put this discovery to practical account. But in 1656 

 Huygens invented the necessary machinery for main- 

 taining the motion of the pendulum and perfected 

 several accurate clocks. These clocks were of in- 

 valuable assistance to the astronomers, affording as 

 they did a means of keeping time "more accurate 

 than the sun itself." When Picard had corrected the 

 variation caused by heat and cold acting upon the 

 pendulum rod by combining metals of different de- 

 grees of expansibility, a high degree of accuracy was 

 possible. 



But while the pendulum clock was an unequalled 

 stationary time-piece, it was useless in such unstable 

 situations as, for example, on shipboard. But here 

 again Huygens played a prominent part by first apply- 

 ing the coiled balance-spring for regulating watches and 

 marine clocks. The idea of applying a spring to the 



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