COMMON THINGS CLOCKS AND WATCHES. 



It must, however, be observed, that generally the hour-lines 

 are inclined to each other at unequal angles, as may be seen by 

 inspecting any ordinary sun-dial. There is ^one, and one only, 

 position which could be assigned to the plate of the dial, such 

 that the hour-lines would make equal angles with each other. 

 That position would be at right angles to the gnomon, and a dial 

 so constructed would be suitable to any place, whatever be its 

 latitude. All that would be necessary would be to set it so that 

 the gnomon would be directed to the celestial pole. The sun, 

 however, would shine upon the upper or north side of it during 

 the spring and summer, and on the lower or south side during the 

 autumn and winter. It would, therefore, be necessary that it 

 should be marked on both sides with hour-lines, and that a 

 gnomon should be fixed on both sides. 



6. The name dial is derived from the Latin word dies, a day, 

 and the invention and use of the instrument as a time indicator is 

 very ancient. According to Herodotus, the invention came to 

 Greece from Chalctaa. The first dial recorded in history is the 

 hemisphere of Berosus, who is supposed to have lived 540 B.C. 



7. The first attempts to measure time by motions artificially 

 produced, consisted in arrangements, by which a fluid was let fall 

 in a continuous stream through a small aperture in the pipe of a 

 funnel, the time being measured by the quantity of the fluid 

 discharged. The CLEPSYDEA, or water-clock, of the ancients, was 

 constructed upon this principle. This and the sun-dial were the 

 only instruments contrived or used by the ancients for the 

 measurement of time. 



Clepsydras were contrived by the Egyptians, and were in 

 common use under the reign of the Ptolemys. In Home, 

 sun-dials were used in summer and clepsydras in winter. These 

 instruments, though subject to very obvious defects, were, never- 

 theless, when skilfully used, susceptible of considerable accuracy, 

 as may be easily conceived, when it is stated, that before the 

 invention of clocks and watches, they were the only chronometric 

 instruments used by astronomers. The chief sources of their 

 irregularities were the unequal celerity with which the fluid is 

 discharged, owing to its varying depth in the funnel and its 

 change of temperature. 



8. The common hour-glass comes under this class of chronometric 

 instruments, but is the most imperfect of them. Nevertheless, for 

 certain purposes, it is even now, advanced as we are in the application 

 of science to the arts, still found the most convenient chronometer. 

 The process of ascertaining a ship's rate of sailing or steaming 

 by means of the log affords ari example of its use. One man holds 

 the reel from which the line runs off, while another holds the 



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