ASTRONOMY. 337 



some state it at 250,000, and others at 252,000 stadia : the length of 

 this unit of measure is also somewhat uncertain. It is, however, of 

 small importance, as we may be pretty well convinced that, by such 

 means as he employed, no very accurate conclusion could be expected; 

 it is sufficient that he attempted the solution of the problem in a very 

 rational manner, to entitle him to the honour of being one of the most 

 celebrated of the Grecian astronomers. 



Eratosthenes also observed the obliquity of the ecliptic, and made Obliquity of 

 it to consist of -reV^h f a circumference, which answers to about l e ec lptlc< 

 23 51' 19 '5". This observation is commonly stated to have been 

 made in the year 230 B. c. 



Archimedes, the justly-celebrated geometer of Syracuse, was con- Archimedes, 

 temporary with Eratosthenes; and although most conspicuous as a B ' c * 

 mechanic and geometrician, the great impulse which he gave to the 

 sciences generally, will not admit of our passing him over in silence 

 in this history. All that we have of this author with reference to 

 astronomy, is found in his ' Arenarius,' a work which has been trans- 

 lated into most modern languages ; where he undertakes to prove, 

 that the numerical denominations which he has indicated in his books 

 to Zeuxippus, are more than sufficient to express the grains of sand 

 that would compose a globe, not only as large as our earth, but as the 

 whole universe. He supposes that the circumference of the earth is 

 not more than three million stadia ; that the diameter of the earth is 

 greater than that of the moon, and less than that of the sun ; that the 

 diameter of the sun is 300 times greater than that of the moon ; and 

 moreover, that the diameter of the sun is greater than the side of the 

 inscribed chiliagon, that is greater than ^Wo, or 21' 36". 



The manner in which he arrives at his conclusion is very interesting, Archimedes 

 as showing the state of the sciences at this time, even in the hands of ?he er a p ent 

 this great master : " I have used," says he, " every effort to deter- diameter of 

 mine, by means of instruments, the angle which comprehends the sun, the sun * 

 and has its summit at the eye of the observer ; but this is not easy ; 

 for neither our eyes nor our hands, nor any of the means which it is 

 possible for us to employ, have the requisite precision to obtain this 

 measure. This, however, is not the place to enlarge upon such a 

 subject. It will suffice to demonstrate that which I have advanced, 

 to measure an angle which is not greater than that which includes the 

 sun's apparent diameter, and has its summit in our eyes ; and then to 

 take another angle which is not less than that of the sun, and which 

 equally has its summit in our eyes. Having, therefore, directed a 

 long ruler on a horizontal plane towards the point of the horizon 

 where the sun ought to rise, I place a small cylinder perpendicularly 

 on this ruler. When the sun is on the horizon, and we look at it 

 without injury, I direct the ruler towards the sun, the eye being at 

 one of its extremities, and the cylinder is placed between the sun and 

 the eye in such a manner, that it entirely conceals the sun from view. 

 I then remove the cylinder farther from the eye, until the sun begins 

 [G. E. P.] z 



