342 THE PRINCIPLES OF SCIENCE. 



A like peculiarity in the motions of Jupiter's fourth satel- 

 lite was similarly detected by Maraldi in 1713. 



Remarkable conjunctions of the planets may sometimes 

 allow us to compare their periods of revolution, through 

 long intervals of time, with great accuracy. Laplace in 

 explaining the long inequality in the motions of Jupiter 

 and Saturn, was much assisted by a conjunction of these 

 planets, observed by Ibyn Jounis at Cairo, towards the 

 close of the eleventh century. Laplace calculated that 

 such a conjunction must have happened on the 3ist of 

 October, A. D. 1087 ; and the discordance between the dis- 

 tances of the planets as recorded, and as assigned by 

 theory, was less than one-fifth of the apparent diameter 

 of the sun. This difference being less than the probable 

 error of the early record, his theory was confirmed as far 

 as facts were available 13 . 



The ancient astronomers often shewed the highest in- 

 genuity in turning any opportunities of measurement 

 which occurred to good account. Eratosthenes, as early 

 as 250 B.C., happening to hear that the sun at Syene, in 

 Upper Egypt, was visible at the summer solstice at the 

 bottom of a well, proving that it was in the zenith, pro- 

 posed to determine the dimensions of the earth, by mea- 

 suring the length of the shadow of a rod at Alexandria on 

 the same day of the year. He thus learnt in a rude 

 manner the difference of latitude between Alexandria and 

 Syene, and finding it to be about one fiftieth part of the 

 whole circumference, he ascertained the dimensions of the 

 earth within about one sixth part of the truth. The use 

 of wells in astronomical observation appears to have been 

 occasionally practised in comparatively recent times, as 

 by Flamsteed in 1679. Hipparchus employed the moon 

 as an instrument of measurement in several sagacious 



b Grant's, 'History of Physical Astronomy,' p. 129. 

 c Baily's, ' Account of Flamsteed,' p. lix. 



