A SILVERED GLASS TELE ST OPE. 



39 



greater than that of solid grains, tliis difKcuhj would not arise. The following 

 contrivance in which the fault of the ordinary clepsydra, in varying rate of flow as 

 the colnmn shortens, is avoided, was next made. With it the best results are 

 attainable, and it seems to be practically perfect. 



The Clepsydra. 



It consists of a cylinder a, in which a piston h moves watertight. At the top of 

 the piston rod is a leaden five-poxmd weiglit c, from which the cord i goes to the 

 sliding platcholdcr </. 'Vhv lower end of the cylinder terminates in a stopcock (/, 

 the handle of which carries a strong index rod e, moving on a lUvided arc. At / 

 a tube with a stopcock is attached. Below, a vessel h receives the waste fluid. 



In using the clepsydra the stopcock of ./' is opened, and the piston being pulled 

 upwards, the cylinder fills with water from Ii. The stopcock is tlien closetl, and if 

 ,1 also is slmt, the weiglit will remain motionless. The string I is next connected 

 with the slide, and the telescope turned on the mooii. As soon as the slide is 

 adjusted in angular position (page 36) the stopcock d is opened, until the weight c 

 moves downwards, at a rate that matches the moon's apparent motion. 



In order to facilitate the rating of the clepsydra, the index rod e is pressed l)y a 

 spring h (2), against an excentric /. As the excentric is turned round, the stojicock 

 d is of course opened, with great precision and delicacy. The plug of this stop 

 cock (3) is not perforated by a round hole, but has a slit. This causes equal move- 



