The sun-dial, by which time was measured by the shadow cast from a pin, rod or pillar upon a graduated horizontal plate--the graduations consisting of twelve equal parts, in which the hours of the day were divided, were, both as to the instrument and the division of the day into hours, invented by the Babylonians or other Oriental race, set up on the plains of Chaldea, constructed by the Chinese and Hindoos--put into various forms by these nations, and adapted, but unimproved, by the learned Greeks and conquering Romans. It appears to have been unknown to the Assyrians and Egyptians, or if known, its knowledge confined to their wise men, as it does not appear in any of their monuments. The clepsydra, an instrument by which in its earliest form a portion of time was measured by the escape of water from a small orifice in the bottom of a shell or vase, or by which the empty vase, placed in another vessel filled with water, was gradually filled through the orifice and which sank within a certain time, is supposed by many to have preceded the invention of the sun-dial. At any rate they were used contemporaneously by the same peoples. In its later form, when the day and night were each divided into twelve hours, the vessel was correspondingly graduated, and a float raised by the inflowing water impelled a pointer attached to the float against the graduations. Plato, it is said, contrived a bell so connected with the pointer that it was struck at each hour of the night. But the best of ancient clepsydras was invented by Ctesibius of Alexandria about the middle of the third century B. C. He was the pupil of Archimedes, and adopting his master’s idea of geared wheels, he mounted a toothed wheel on a shaft extending through the vessel and carrying at one end outside of the vessel a pointer adapted to move around the face of a dial graduated with the 24 hours. The vertical toothed rod or rack, adapted to be raised or lowered by a float in a vessel gradually filled with water, engaged a pinion fixed on another horizontal shaft, which pinion in turn engaged the larger wheel. It was not difficult to proportion the parts and control the supply of water to make the point complete its circuit regularly. Then the same inventor dispensed with the wheel, rack, and pinion, and substituted a cord to which a float was attached, passing the cord over a grooved pulley and securing a weight at its other end. The pulley was fixed on the shaft which carried the hour hand. The float was a counterbalance to the weight, and as it was lifted by the water the weight stretched the cord and turned the pulley, which caused the pointer to move on the dial and indicate the hour. The water thus acted as an escapement to control the motive power. In one form the water dropped on wheels which had their motion communicated to a small statue that gradually rose and pointed with a rod to the hour upon the dial.