110 Scientific Proceedings, Royal Dublin Society. 



with each other, into which wheels a pinion p is geared, this pinion 

 being carried on a stud fixed to a disc (e) revolving free on shaft : 

 when not in use the pinion acts as a clutch, and both wheels and 

 shaft, pinion and disc, all revolve together by clock. When a 

 fine motion is required in either direction, a cord passing over a 

 groove in disc is pulled in one direction or other ; the pinion 

 revolves round differential wheels, and as the wheels have not 

 exactly the same number of teeth, produces a differential motion 

 which practically accelerates or retards clock-movement as long as 

 cord is pulled, but the moment the cord is released all revolve 

 together, as before, at normal rate. 



The only objection that has ever been made to this motion is, 

 that it requires two hands to work it — one to keep the cord a little 

 "taut," and the other to pull, as otherwise the cord would slip 

 round without gripping. There is more difficulty in overcoming 

 this objection than may at first sight be apparent, for it will not 

 answer to apply anything which will produce friction between cord 

 and pulley, except at the moment the cord is pulled. 



The following plan has, however, proved quite successful : — 



A little frame {ad) is fixed over the pulley disc in such a manner 

 that it is capable of a rocking motion in centre [a). This frame 

 carries four rollers (b, b', c, c) . The pulleys (b, b') are simple grooved 

 rollers ; (c, c') are covered with india-rubber rings. When cord k is 

 pulled, the whole frame slightly tilts, and brings the rubber-covered 

 roller (c r ) into good contact with cord on disc pulley, and prevents 

 it slipping. When cord It is pulled the other rubber-covered roller 

 (c) is brought into contact. The moment either end of cord is 

 released the rollers return to their normal position out of contact. 

 As the roller which is brought into contact is nearly a whole cir- 

 cumference from the point where the cord is let off, the cord has a 

 good grip on the pulley, and never slips. 



Mew Slow Motion in Right Ascension. — The recent 

 advances in celestial photography have rendered it desirable to 

 have a more delicate and accurate slow motion in Right Ascen- 

 sion than has hitherto been required. The necessity for this is 

 partly due to the fact that up to the present no clock-work has 

 been found sufficiently accurate to keep the star absolutely steady 

 on the photo plate for the long period necessary to obtain an 

 image of faint stars; and consequently it has been the practice 



