176 EPOCHS IN THE HISTORY OF THE CONTEMPLATION 



apparition of a new star ( 273 ) )to this great discovery; to winch 

 the long-continued observation of the heliacal rising of Sirius 

 ought indeed to have conducted the earlier Egyptians. ( 2 7 4 ) 

 Another peculiar feature in the proceedings of Hippar- 

 chus, was his endeavouring to avail himself of celestial phe- 

 nomena for determinations of geographical position. Such 

 a combination of the study of the heavens and the earth, 

 the knowledge of the one becoming reflected on the other, 

 served by its uniting tendency to give a lively impulse to 

 the great idea of the Cosmos. In a new map of the world, 

 constructed by Hipparchus, and founded on that of Eratos- 

 thenes, wherever the application of astronomical observa- 

 tions was possible, the geographical positions were assigned 

 by longitudes and latitudes, obtained, the former from lunar 

 eclipses, and the latter from lengths of the solar shadow 

 measured by the gnomon. The hydraulic clock of Ctesibius, 

 an improvement upon the ancient Clepsydra, might afford 

 the means of making more exact measurements of time ; 

 whilst, for determinations in space, gradually improved 

 means of angular measurement were offered to the Alexan- 

 drian astronomers, from the old gnomon and scaphe to the 

 invention of astrolabes, solstitial armills, and dioptras. 

 Thus men arrived by successive steps, as if by the acquisi- 

 tion of new organs, to a more exact knowledge of the 

 movements of the planetary system. It was only the know- 

 ledge of the absolute magnitudes, forms, masses, and phy- 

 sical constitution of the heavenly bodies, which made no 

 progress for many centuries. 



f" Not only were several practical astronomers of the Alex- 

 andrian school themselves distinguished geometricians, but 

 the epoch of the Ptolemies was moreover the most brilliant 



