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SCIENCE 



[N. S. Vol. XXV. No. 638 



wMcli the water by slow degrees finds its 

 way into the interior. When the shell fills 

 and sinks, the man on watch calls the time 

 and sets it afloat again. 



The gradual development through un- 

 told ages of the floating copper vessel has 

 given us our finest astronomical clocks of 

 to-day, and similarly the floating cocoanut 

 shell may be considered the simplest form 

 of a marine chronometer. 



If now we visit the Chinese Empire we 

 find in use there also the water clock, but 

 instead of the water flowing into a vessel 

 through a small hole in the bottom, it flows 

 out of the hole. In attempting to calibrate 

 such a vessel so that any given portion of 

 the whole time required to empty it might 

 be determined, it is at once noticed that 

 when the vessel is nearly full the water 

 flows out more rapidly than when it is 

 nearly empty. The difficulty here pre- 

 sented is most easily overcome by keeping 

 the vessel filled with water when the flow 

 will be uniform, and catching the dis- 

 charged water in a cylindrical vessel in 

 which the surface of the water will rise 

 equal distances in equal times. Chinese 

 writers ascribe the invention of this instru- 

 ment, called a clepsydra, to Hwang-ti, who 

 lived more than twenty-flve centuries be- 

 fore our era. About fourteen centuries 

 later Duke Chau introduced a float upon 

 the surface of the water in the final cyl- 

 inder by means of which an indicator was 

 made to travel over an adjacent scale as 

 the water rose in the cylinder, thus allow- 

 ing the indications of the instrument to be 

 perceived at a greater distance. 



These instruments that have just been 

 described may be called artificial time- 

 keepers, and are used primarily to sub- 

 divide the day, while the sundial of equal 

 antiquity with the others may be called a 

 natural timekeeper, as it gives a means of 

 determining day after day a particular 

 time of day, such as apparent noon. 



The earliest mention of a sundial is 

 found in Isaiah 38: 8, in connection with 

 the promise of the Lord to add fifteen years 

 to the life of Hezekiah who lived about 

 2,600 years ago. "And this shall be a sign 

 unto thee from the Lord, that the Lord 

 will do this thing that he hath spoken ; be- 

 hold, I will bring again the shadow of the 

 degrees, which is gone down in the sundial 

 of Ahaz, ten degrees backward. So the sun 

 returned ten degrees, by which degrees it 

 Was gone down." 



Of the nature of this sundial nothing is 

 said, nor is there foimd any description of 

 such an instrument until 350 years later, 

 when we find the sundial of the Chaldean 

 priest Berosus, who lived in the time of 

 Alexander the Great and his immediate 

 successors. 



This consisted of a hollow hemisphere 

 placed with its rim perfectly horizontal, 

 and having a bead fixed at its center. So 

 long as the sun remained above the horizon 

 the shadow of the bead would fall on the 

 inside of the hemisphere, and the path of 

 the shadow during the day would be ap- 

 proximately a circular arc. This arc, di- 

 vided into twelve equal parts, determined 

 twelve equal intervals of time for that day. 

 Now supposing this were done at the time 

 of the solstices and equinoxes, and on as 

 many intermediate days as might be con- 

 sidered sufScient, and then curve lines 

 drawn through the corresponding points of 

 division of the different ares, the shadow 

 of the bead falling on one of these curve 

 lines would mark a division of time for 

 that day, and thus we should have a sun- 

 dial which would divide each period of 

 daylight into twelve equal parts. These 

 equal parts were called temporary hours; 

 and since the duration of daylight varies 

 from day to day, the temporary hours of 

 one day would differ in length from those 

 of another. Dials of this form were still 

 used by the Arabians a thousand years ago, 



