422 A Clepsydra for Driving Telescopes. 



scope. The curved bracket has also inserted a pulley, P, the 

 frame of which turns freely on its axis. 



Inside the vessel A is a float, F, also made in zinc, six 

 inches long, and three and a half inches diameter, air-tight. 

 In the bottom of it is a disc of lead, weighing two pounds, 

 which suffices to depress the float to the level of the water 

 without sinking it. A fine string, t t, is attached to a wire 

 loop in the float, and is carried over the pulley, and thence to 

 the telescope. 



As the force to be exerted by the float F, in moving the 

 telescope, is not greater than that represented by its weight, 

 some mechanical advantage may be gained by attaching the 

 string to the end of the telescope tube, near the eye-piece, 

 rather than to its frame, or to the polar axis. My own tele- 

 scope, etc., although it weighs 250 pounds, is in this manner, 

 when clamped in declination, drawn with the utmost facility 

 by the float weighing only two pounds. 



Since the accompanying sketch was made two conveniences 

 have been applied which are not here shown. One of these 

 is a " tell tale/ J which is drawn up outside the vessel A, by a 

 string connecting it with the descending float ; this serves to 

 show how much water remains in the vessel. The other is a 

 ready way of regulating the tap in the syphon-tube, when the 

 apparatus is placed out of arm's reach, as it often may be. 



It was most convenient in my own case to hang the appa- 

 ratus against one side of the moveable roof of the observatory, 

 at right angles to the opening. In this position the tap was 

 five feet distant, immediately behind me, when at the eye- 

 piece. To remedy this inconvenience I attached a small lever 

 to the tap, and carried a string from it through a pulley over 

 head in the roof, and brought it down over another pulley to 

 the right hand of my station. By attaching an equal weight 

 to either end of the string the tap could be kept open at any 

 degree that might be required, and regulated with great pre- 

 cision without my leaving the eye-piece. 



To put the apparatus in action let the vessel A be first 

 placed exactly vertical, so that the float in descending may not 

 rub against the sides ; fill the vessel with water until the float 

 rises, and is just visible at the top ; suck the air out of the 

 syphon-tube, and when the water follows turn the tap to arrest 

 its flow. The string is then tied to the float F, brought over 

 the pulley P, and slightly secured to the telescope. Let a 

 star be placed in the field, and the string somewhat tight- 

 ened. In doing this, the probability is, that the instru- 

 ment, being pulled slightly, the star will escape out of 

 the field, apparently eastward, if it be a reflector. The 

 instrument may then be allowed to rest until the star re- 



