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Greeks, from whom it passed to the Romans. This is inferred 
from passages in the works of Aristotle.* The Clepsydra which 
stood in the Circus Maximus, at Rome, is represented on the gem 
as a large vase, held up by two winged boys, from the bottom of 
which the water is flowing. The urn, or vase, has upon it a 
horse at full speed.+ 
About two centuries before the Christian Era Ctesibius, a 
Mathematician of Alexandria, who had studied the principles of 
hydraulics, invented the /Vater-clock. 
This consisted of a cylindrical vessel filled with water, bearing 
up a float which rose and fell with the water; and there was a 
vertical gauge marked wiih the hours; the float by its gradual 
ascent, as the water entered through an opening in the cylinder, 
showed the passing of the time. 
Vetruvius describes a Clepsydra, which not only told the hours 
of the day, but the moon’s age, the zodiacal sign of the month, 
and acted as an astronomical clock.t The mechanism of this was 
a great step in advance, and, as Mr. King observes, “‘renders it 
probable that the common Roman Clepsydra had a dial-face and 
a pointer, or hand, set in motion by a string and float, 
and acted like the index of our wheel barometers. In the 
Horologium Anaphorium of Vetruvius, the dial painted 
with the World and the Zodiac, was traversed by an axis on 
which was wound a flexible brass chain supporting by its one end 
the float, on the other a balance weight, equal to that of the 
float, as the latter rose with the water, so the balance weight 
descending, unwound the chain and made the dial revolve.” 
* Aristotle. Problem xvi. 8. 
+ Cesar mentions the Clepsydra in his Commentaries, Book V. 13, in 
speaking about the length of the day in Britain, and that by its 
measurements the nights seemed shorter than on the continent. 
“ Certis ex aqua mensuris breviores esse quam in continenti noctes 
videbamus.” 
t+ See Vetruvius, lib. ix., c. viii, 
