122 THE WORLD MACHINE 



the coast of ^Egina, but JEgina. itself, would be laid completely 

 under water." l 



And Strabo laughs. Is this Strabo some philosopher of the 

 days, say, of Elizabeth or Louis Quatorze, sneering at the 

 wisdom of the ancients ? No, somewhat further back back 

 even to the time when they were setting a crown on the head 

 of the first Imperator of Rome. And this cheery old Strabo, 

 writing his celebrated geography, if we may trust Humboldt, 

 when he had passed eighty, goes on to reprobate these ideas 

 of the Alexandrian as plain nonsense. These currents in the 

 Mediterranean, between Scylla and Charybdis, and elsewhere, 

 which Eratosthenes adduces, are, he says, the effect of the tides ; 

 and the tides and their dependence on the rising and the setting 

 of the moon, have been sufficiently treated by Poseidonius and 

 Athenodorus, so that he does not need to go into the matter. 

 So the flat misprisions of his predecessor seem to him simply 

 absurd. " Whoever," he exclaims, " imagined the surface of 

 the ocean to be on a slope ? For water is not like the earth, 

 which, being of a solid nature, is capable of permanent depres- 

 sions and risings, but by the force of gravity spreads equally over 

 the earth, and assumes that kind of level which Archimedes 

 assigned to it ! " 



Thus Strabo, A.D. 25. In Poseidonius, a full century before, 

 and in Archimedes, yet another century back of him, we find 

 clear enough conceptions that the oceans lie at a general level 

 over the surface of a spherical earth, perforce of a power directed 

 towards the earth's centre. That gravity draws always to the 

 centre, seems to have been present, hazily, in the hazy mind of 

 Aristotle ; it was clearly one of the established verities in the 

 Alexandrian school a century later. Strabo even makes of it 

 an argument for the sphericity of the earth, since " all things 

 however distant, tend toward its centre." A weight hung on 

 the end of a string is always perpendicular to the plane of the 

 horizon, that is, to a tangent to the earth's surface, anywhere. 

 If the earth is a sphere, it follows that the plumb-line points 

 always to the centre of the sphere. 



Bodies falling from no matter what height fall always in the 



same line ; they form a perpendicular to a water level. This 



the ancients knew well. It was as if there is an attractive force, 



acting from the earth's centre. And this force seemed constant, 



1 Strabo, Geography, i. p. 85 ; ed. Bohn. 



