94 THE WORLD MACHINE 



make the distance of the sun a thousand times that of the moon, 

 an estimate two and a half times too great ; but in the face 

 of his predecessor's figure of twenty times what a sublime and 

 daring error ! 



It is with a deepening interest, bordering even upon amaze- 

 ment, that we find yet another great investigator of antiquity 

 announcing similar but quite distinct estimates. This was 

 Poseidonius, the teacher of Cicero and of Pompey, one of the 

 most contradictory of characters, now seeming but a merest 

 polymath, now one of the most acute and original thinkers 

 of that ancient day. We have already noted that his measure 

 of the earth, adopted by Ptolemy, was the sustenance of 

 Columbus. He had closely studied the refraction of light, 

 and gives us a really wonderful calculation as to the height 

 of the earth's atmosphere. In the pages of Cleomedes we 

 learn that he equally attempted to establish the distance of 

 the stars. He puts the moon at two million stadia away, the 

 sun at five hundred million ! This, on his earlier estimate of 

 the earth's diameter, would place the moon at fifty-two radii 

 of the earth, which would be nearer than the computations 

 of Hipparchus. It would make the sun's distance 13,000 radii. 



If we take his later figure (180,000 stadia), the distance 

 would become 17,400 radii, an estimate which, considering the 

 necessarily wide limits of error, does not differ greatly from 

 that of Eratosthenes, and equally little from the truth. Com- 

 pare it with the thirteen hundred radii of his forerunners ! 

 Compare it with the notions of Epicurus, almost his contem- 

 porary, a very wise and large-minded man in his way, who yet 

 believed that the sun might be a body two feet across ! 



By what means Poseidonius reached these astonishing com- 

 putations, what instruments he had, we do not know, or 

 whether his method was the same as that of Eratosthenes. 

 His own works have perished, though those of his friend Cicero 

 have survived ; and Cleomedes, who has preserved his figures, 

 is silent. By means of a clepsydra, or water-clock, he had 

 computed the sun's disk at a little over twenty-eight minutes 

 of arc, whence he calculated the actual diameter at four million 

 stadia, or seventy times the diameter of the earth. Consider 

 his age, and you will realise how intrepidly he followed whither- 

 soever his calculations carried, even to such unbelievable con- 



