CHAPTER X 



ARCHIMEDES AND THE FIRST IDEAS OF 

 GRAVITATION 



THE earth is round, but water flows. Why do not the oceans 

 run down hill and spill away into the void ? 



The cause, the " reason," seems so obvious to us now that 

 it is a little difficult to realise that this was once a puzzle and 

 stumbling-block. Let us go back and consider the problem 

 with the earliest of the physiographers. One of Jthese was that 

 same Eratosthenes whom we have seen measuring the circum- 

 ference of the earth, the tilt of its axis to the plane in which 

 the sun revolves ; computing the distance of the sun with 

 greater success than any investigator of antiquity ; a giant 

 mind, and, moreover, one of those finely trained imaginations 

 which seems able to escape the enveloping trammels of accus- 

 tomed methods and beliefs, to reach a larger view. But no 

 man can shake himself wholly free from the web of his own time ; 

 so we find in Strabo such an extraordinary passage as this : 



" However, so nice a fellow is Eratosthenes, that though he 

 professes himself a mathematician, he rejects entirely the dictum 

 of Archimedes, who, in his work, On Bodies in Suspension, 

 says that all liquids, when left at rest, assume a spherical form, 

 having a centre of gravity similar to that of the earth ; a dictum 

 which is acknowledged by all who have the slightest pretensions 

 to mathematical sagacity. He says that the Mediterranean, 

 which, according to his own description, is one entire sea, has 

 not the same level even at points quite close to each other ; 

 and offers us the authority of engineers for this piece of folly. 

 He tells us that Demetrius intended to cut through the Isthmus 

 of Corinth to open a passage for his fleet, but was prevented by 

 his engineers, who, having taken measurements, reported that 

 the level of the sea at the Gulf of Corinth was higher than the 

 opposite side, so that if he cut through the Isthmus, not only 



