38 Scientific Proceedings, Royal Dublin Society. 
this accelerates or retards (according to the manner in which the 
wheels are arranged) the rate of the driving-screw. This slow 
motion is exceedingly fine, and only used for the most delicate 
setting. 
For a less delicate setting an arrangement has for many years 
been used by which a motion, backward or forward, can be given 
to one of the loose discs carrying the pinions, and thus a less 
delicate slow motion is provided. But even this is very slow 
when it is desired to move the image of the star any considerable 
distance across the field of view; and therefore, in the most 
recent examples, I have found the following arrangement a great 
convenience. 
The edge of one of the loose discs is cut into fine teeth, or 
preferably, coarsely milled ; and a small electric motor is supplied 
to drive this disc very quickly round in one direction or another. 
The electric motor is started, stopped, or reversed by a commu- 
tator held in the observer’s hand, and connected by a flexible cord 
to the necessary circuits; and the arrangement is such that, when 
at rest, the pinion which drives the dise (which is generally made 
of leather) is completely free from the disc itself, but is brought 
into gear by a special electric magnet, which is not excited 
until the motor itself has gained a high speed. ‘Thus the motor 
starts without load—an important point. 
In practice it is not found desirable to pass the compensating 
heavy current that is used to drive the motor through the long and 
fine flexible cords which connect the commutator in the observer’s 
hand with the instrument. 
A small current passes from the commutator to a set of 
relays fixed in any convenient place; and the relays when in 
action close the proper circuit with the heavy current, and 
drive the motor forward or backward as desired. 
