A Clepsydra for Driving Telescopes. 423 



turns, and as soon as it gets fairly into the field, the tap of 

 the syphon must be turned, causing the water to flow. It may 

 be a few seconds before the string tightens to lift the float, 

 the weight of which has to overcome the resistance of the 

 telescope, etc.; but when that has once taken place, and the 

 telescope begins to follow, the regulation of the water-flow in 

 obtaining the sidereal rate is easy. If the star has got across 

 the field by the time the float is in full action, it is evident the 

 pace of the telescope must be increased, in order that the star 

 may be overtaken; and this may be instantly effected by 

 opening the tap wider, and increasing the flow of water. When 

 the star is overtaken, and it commences moving across the field 

 apparently eastwards, as before, the flow must be gradually 

 diminished, until the star is " brought up." With a very little 

 practice, the sidereal rate may be so closely approximated to, 

 that the star can be made to remain motionless at the centre 

 of the field, and will, of course, continue there until the tube 

 is exhausted of water. In a tube four inches diameter the 

 sidereal rate is represented by a drop of one inch in four 

 minutes and a half; and, therefore, with a tube thirty- six 

 inches long, deducting six inches for the float, an object may 

 be kept in the field of view 135 minutes, a period of time 

 more than sufficient for any single observation. I need scarcely 

 point out that the tube may be put in action again imme- 

 diately by pouring all the water back into it. 



I by no means intend to convey the impression that this 

 apparatus is equal in point of convenience to a fine clock 

 motion, but a most excellent substitute for clock motion it 

 certainly is, and I can therefore confidently recommend it to 

 amateurs who, having equatoreal arrangements, are destitute 

 of a driving force. The cost of the apparatus is small, and 

 it can be made by any tinsmith. If the foregoing description 

 should not appear very intelligible, it will afford me pleasure 

 to answer any questions, or even to get the apparatus made 

 complete for any one who may wish to try it. 



General Cemetery, Birmingham, Nov. 22, 1865. 



