* 155158] GaScoigilt) Ilorro.ks, Huygens 203 



of Philips von Lansberg (1561-1632), which Hurrocks had 

 verified for the purpose. It was not, however, till long 

 afterwards that Halley pointed out the importance of the 

 transit of Venus as a means of ascertaining the distance of 

 the sun from the earth (chapter x., 202). It is also worth 

 noticing that Horrocks suggested the possibility of the 

 irregularities of the moon's motion being due to the disturb- 

 ing action of the sun, and that he also had some .idea of 

 certain irregularities in the motion of Jupiter and Saturn, 

 now known to be due to their mutual attraction (chap er x., 

 204 ; chapter XL, 243). 



157. Another of Huygens's discoveries revolutionised the 

 art of exact astronomical observation. This was the inven- 

 tion of the pendulum-clock (made 1656, patented in 1657). 

 It has been already mentioned how the same discovery 

 was made by Biirgi, but virtually lost (see chapter v., 98) ; 

 and how Galilei again introduced the pendulum as a time- 

 measurer (chapter vi., 114). Galilei's pendulum, however, 

 could only be used for measuring very short times, as there 

 was no mechanism to keep it in motion, and the motion 

 soon died away. Huygens attached a pendulum to a clock 

 driven by weights, so that the clock kept the pendulum going 

 and the pendulum regulated the clock.* Henceforward 

 it was possible to take reasonably accurate time-observa- 

 tions, and, by noticing the interval between the passage 

 of two stars across the meridian, to deduce, from the known 

 rate of motion of the celestial sphere, their angular distance 

 east and west of one another, thus helping to fix the position 

 of one with respect to the other. It was again Picard ( 155) 

 who first recognised the astronomical importance of this 

 discovery, and introduced regular time-observations at the 

 new Observatory of Paris. 



158. Huygens was not content with this practical use 

 of the pendulum, but worked out in his treatise called 

 Oscillatorium Horologium or The Pendulum Clock (1673) a 

 number of important results in the theory of the pendulum, 

 and in the allied problems connected with the motion of 

 a body in a circle or other curve. The greater part of these 



* Galilei, at the end of his life, appears to have thought of contriving 

 a pendulum with clockwork, but there is no satisfactory evidence that 

 he ever carried out the idea. 



