26 PROGRESS OF SCIENCE. 



to our thinking the most marvellous feat of scientific genius 

 ever performed. It was reserved to Newton to show the 

 cause of this phenomenon. 18, Hipparchus is said by some 

 authors to have also understood and explained our SOLAR 

 SYSTEM i.e. the heliocentric theory, revived and demon- 

 strated by Copernicus. It is probable that he thought like 

 Aristarchus on the subject. 



135 51. Posidonius measured a degree; and, having 

 observed the intervals between successive high TIDES to be 

 equal to the intervals between the moon's passage across the 

 meridian, correctly ascribed the Tides to be due to the 

 motions of the sun and the moon. Pliny, who came to the 

 same conclusion, and who stated it, only recorded or repeated 

 what Posidonius had observed. 



100 45. Sosigenes, under the direction of Julius Caesar, 

 reformed the Roman Calendar, and devised the JULIAN 

 CALENDAR (45 B.C. 1581-2 A.D.) 



B.C. 54 24 A.D. Strabo was a traveller and geographer; 

 he described all he saw with remarkable accuracy ; studied 

 VOLCANOES and earthquakes; pointed out that these phe- 

 nomena are caused by the hot vapour hidden within the earth ; 

 that when the vapour and lava can find a valve of escape as 

 they do in the Etna, the crust of the earth is not disturbed ; but 

 when they cannot escape they cause shocks and earthquakes. 



B.C. 5 65 A.D. Seneca asserted that comets would be 

 found to revolve in PERIODIC ORBITS and return to sight 

 the Chaldaeans and the Pythagorians (if not Pythagoras 

 himself) entertained a like opinion. He knew that the tails 

 of comets are driven away from the sun ; and like Anaxagoras 

 thought Meteorites must be cosmical bodies. 



100161 A.D. Ptolemy discovered the EVECTION OF THE 

 MOON ; determined the moon's parallax, and made several 

 other discoveries of secondary importance relating to the 

 motions of the moon and the planets ; invented the stereo- 

 graphic projection and the quadrant. His SYNTAXIS contains 

 the roots of most of the methods in use among us. He made 

 a regular system of astronomy based upon all the knowledge 

 the Greeks had acquired about the heavenly bodies, and this 

 system was accepted for 1,400 years until Copernicus. His 



