2O4 A Short History of Astronomy [Cn. viil. 



investigations lie outside the field of astronomy, but his 

 formula connecting the time of oscillation of a pendulum 

 with its length and the intensity of gravity * (or, in other 

 words, the rate of falling of a heavy body) afforded a prac- 

 tical means of measuring gravity, of far greater accuracy 

 than any direct experiments on falling bodies; and his 

 study of circular motion, leading to the result that a body 

 moving in a circle must be acted on by some force towards 

 the centre, the magnitude of which depended in a definite 

 way on the speed of the body and the size of the circle,t is 

 of fundamental importance in accounting for the planetary 

 motions by gravitation. 



159. During the iyth century also the first measurements 

 of the earth were made which were a definite advance on 

 those of the Greeks and Arabs (chapter n., 36, 45, 

 and chapter in., 57). Willebrord Snell (1591-1626), best 

 known by his discovery of the law of refraction of light, 

 made a series of measurements in Holland in 1617, from 

 which the length of a degree of a meridian appeared to be 

 about 67 miles, an estimate subsequently altered to about 

 69 miles by one of his pupils, who corrected some errors 

 in the calculations, the result being then within a few 

 hundred feet of the value now accepted. Next, Richard 

 Norwood '(1590 ?-i675) measured the distance from London 

 tc York, and hence obtained (1636) the length of the 

 degree with an error of less than half a mile. Lastly, 

 Picard in 1671 executed some measurements near Paris 

 leading to a result only a few yards wrong. The length 

 of a degree being known, the circumference and radius of 

 the earth can at once be deduced. 



1 60. Auzout and Picard were two members of a group 

 of observational astronomers working at Paris, of whom the 

 best known, though probably not really the greatest, was 

 Giovanni Domenico Cassini (1625-1712). Born in the 

 north of Italy, he acquired a great reputation, partly by 

 some rather fantastic schemes for the construction of 

 gigantic instruments, partly by the discovery of the rotation 



* In modern notation : time of oscillation = 2 IT ^ljg> 

 f I.e. he obtained the familiar formula v*/r, and several equivalent 

 forms for centrifugal force*. 



