AIDS OF THE NEWTONIAN PERIOD. 275 



mentioned, has invented a method of dividing the 

 circle still superior to the former ones; indeed, 

 one which is theoretically perfect, and practically 

 capable of consummate accuracy. In this way, 

 circles have been constructed for Greenwich, Ar- 

 magh, Cambridge, and many other places ; and pro- 

 bably this method, carefully applied, offers to the 

 astronomer as much exactness as his other imple- 

 ments allow him to receive; but the slightest 

 casualty happening to such an instrument, or any 

 doubt whether the method of graduation has been 

 rightly applied, makes it unfit for the jealous scru- 

 pulosity of modern astronomy. 



The English artists sought to attain accurate 

 measurements by continued bisection and other 

 aliquot subdivision of the limb of their circle ; but 

 Mayer proposed to obtain this end otherwise, by 

 repeating the measure on different parts of the cir- 

 cumference till the errour of the division is unim- 

 portant, instead of attempting to divide an instru- 

 ment without errour. This invention of the Re- 

 peating Circle was zealously adopted by the French, 

 and the relative superiority of the rival methods is 

 still a matter of difference of opinion (v). 



2. Clocks. The improvements in the measures 

 of space require corresponding improvements in the 

 measure of time. The beginning of anything which 

 we can call accuracy, in this subject, was the appli- 

 cation of the Pendulum to clocks, by Huyghens, in 

 1656. That the successive oscillations of a pen- 



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