276 HISTORY OF PHYSICAL ASTRONOMY. 



dulum occupy equal times, had been noticed by 

 Galileo; but in order to take advantage of this 

 property, the pendulum must be connected with 

 machinery by which its motion is kept from lan- 

 guishing, and the number of its swings recorded. 

 By inventing such machinery, Huyghens at once 

 obtained a measure of time more accurate than the 

 sun itself. Hence astronomers were soon led to 

 obtain the right ascension of a star, not directly, by 

 measuring a Distance in the heavens, but indirectly, 

 by observing the Moment of its Transit. This ob- 

 servation is now made with a degree of accuracy 

 which might, at first sight, appear beyond the limits 

 of human sense, being noted to a tenth of a second 

 of time : but we may explain this, by remarking 

 that though the number of the second at which the 

 transit happens is given by the clock, and is reck- 

 oned according to the course of time, the subdivision 

 of the second of time into smaller fractions is per- 

 formed by the eye, by seeing the space described 

 by the heavenly body in a whole second, and hence 

 estimating a smaller time, according to the space 

 which its description occupies. 



But in order to make clocks so accurate as to 

 justify this degree of precision, their construction 

 was improved by various persons in succession. 

 Picard soon found that Huyghens' clocks were 

 affected in their going by temperature, for heat 

 caused expansion of the metallic pendulum. This 

 cause of errour was remedied by combining different 



