PROGRESS IN ANTIQUITY. 23 



HYDROSTATICS by the discovery of the principle which bears 

 his name: "A body immersed in a liquid loses a part of its 

 weight equal to the weight of the displaced liquid." This law 

 is the basis among other things of the calculations in ship- 

 building. And the same principle is true for bodies in air ; 

 so that Archimedes made the first successful attempt to 

 measure the SPECIFIC GRAVITY of different substances. 



285 222. Hero (of Alexandria), one of the great 

 mathematicians of the time, made several mechanical in- 

 ventions, one of which was the pneumatic experiment called 

 HERO'S FOUNTAIN, in which a jet of water is maintained 

 by condensed air ; another, a STEAM-ENGINE the reaction 

 machine (the Eolipile), of which we have the description ; 

 also a double forcing pump used for a FIRE-ENGINE. 



280 264. Aristarchus (of Samos) attempted to measure 

 the distance from the earth to the sun and to the moon 

 by a most ingenious method (dichotomy) ; measured the 

 magnitude of the sun's diameter ; calculated the year to be 

 365 days 6 hours ; held the HELIOCENTRIC DOCTRINE (also 

 ascribed to Pythagoras and Philolaus), teaching that it was 

 the earth, not the sun, which travelled round the ecliptic. 

 He understood that the position of the earth from pole to 

 pole is sloping, that is, oblique, to the ecliptic, and that this 

 OBLIQUITY causes the four seasons of the year. He appa- 

 rently was the first among the Greeks who dearly understood 

 that the change from day to night, and from night to day, 

 is due to the turning of the earth on its axis in 24 hours. 



276 196. Eratosthenes made use of the solstitial 

 and equinoctial ARMILS (armillary spheres) which he invented, 

 or received from Ptolemy Euergetes, by means of which he 

 determined the distance between the tropics to be equivalent 

 to 47 42' 39", half of which gives 23 51' 19"'$ for the 

 OBLIQUITY OF THE ECLIPTIC. (In 1 870 it was calculated to 

 be 23 27' 22"'3 and is continually diminishing the rate of 

 decrease having been some twenty minutes per thousand 

 years.) We need hardly point out the great importance of this 

 discovery. Further, by his innovations, Eratosthenes may be 

 considered as the FATHER OF GEOGRAPHY. He constructed 

 a map of the known world ; described Libya, Asia, Europe ; 



