Grubb — Improvements in Equatorial Telescope Mountings. Ill 



for the operator to have a very powerful finder-telescope with 

 cross-lines set on a particular star, and to watch this star during 

 the progress of exposure, and if he saw it vary its position by 

 the smallest quantity to bring it back again by the slow motion 

 of the instrument. Anyone who has experience in these matters 

 will know how very difficult it is to obtain a slow motion which is 

 perfectly certain in its action, has no back lash, and acts promptly, 

 without and at same time setting the instrument into swing. The 

 new slow motion which the writer has devised is not subject to 

 these faults, and may thus be described : — 



In the slow motion by differential wheels, described above, it is 

 evident that if disc carrying pinion be simply stopped, a retarda- 

 tion or acceleration (according to relative positions of wheels) will 

 be produced, to a slight extent. Suppose the wheels to have twenty- 

 nine and thirty teeth, the speed will be altered -^ part quicker or 

 slower. Now, suppose two such differential sets of wheels be placed 

 side by side on the shaft (which of course should be cut in two 

 places), but with wheels so arranged that a stopping of one disc 

 (and pinion) will produce an acceleration of 3^, and the stopping 

 of the other will produce a retardation of -g 1 ^. A pair of electro- 

 magnets working a clutch (or simple levers with strings) can be 

 used to enable the operator, who has his eye applied to finder, to 

 bring either the retarder or accelerator into action at any instant ; 

 and the action is so gentle that no swing or disturbance of the 

 instrument takes place. 



