A SILVERED GLASS TELESCOPE. 



39 



greater than that of solid grains, this difficulty would not arise. The following 

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

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

 attainable, and it seems to be practically perfect. 



Fig. 35. 



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-pound weight c, from which the cord i goes to the 

 sliding platehold^r g. The lower end of the cylinder terminates in a stopcock d, 

 the handle of which carries a strong index rod e, moving on a divided 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 h. The stopcock is then closed, and if 

 d also is shut, the weight will remain motionless. The string t is next connected 

 with the sHde, and the telescope turned on the moon. 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 by a 

 spring Tc (2), against an excentric I. As the excentric is turned round, the stopcock 

 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- 



