The various approaches to the citadel on the northern side - the rock-cut flight of steps north-east of the Erechtheum, the stairs leading to the well Clepsydra, and the intermediate passage supposed to have furnished access to the Persians - are all to be attributed to the primitive epoch.
The Pelasgic wall enclosed the spring Clepsydra, beneath the north-western corner of the Acropolis, which furnished a watersupply to the defenders of the fortress.
On the north-west rock the caves known as the grottoes of Pan and Apollo were cleared out; these consist of a slight high-arched indentation immediately to the east of the Clepsydra and a double and somewhat deeper cavern a little farther to the east.
1070 I 060 960 950 930 911 889 883 858 825 823 810 781 771 753 745 727 722 705 681 668, of squares and cubes, calculated from 1 to 60, have been found at Senkera, and a people who were acquainted with the sun-dial, the clepsydra, the lever and the pulley, must have had no mean knowledge of mechanics.
The first, for a memoir on the construction of a clepsydra for measuring time exactly at sea, he gained at the age of twenty-four; the second, for one on the physical cause of the inclination of the planetary orbits, he divided with his father; and the third, for a communication on the tides, he shared with Euler, Colin Maclaurin and another competitor.