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XV.— NOTE ON SOME IMPEOVEMENTS IN EQUATORIAL 

 TELESCOPE MOUNTINGS. By HOWARD GRUBB, F.R.S. 



[Read, January 20, 1886.] 



lew Declination Slow Motion. — The slow motion arrange- 

 ments usually used in Equatorials are of either of two forms, viz. : — 



(a) an endless screw working into a sector or portion of a 

 toothed circle of long radius ; or, 



(b) A screw applying, or pushing directly against an arm, that 

 arm being kept in contact with the screw by a spiral or some other 

 form of spring having a considerable range of motion. 



The first (a) possesses the disadvantage that, however carefully 

 made, it is impossible it is quite free from " loss" or " back lash " ; 

 and, consequently, the position of the telescope is not perfectly 

 determinate in declination, which fault is inconvenient when deli- 

 cate measures are required. 



The second (b) has practically no " back lash," as spring keeps 

 the arm in perfect contact with screw, but it has the disadvantage, 

 that whatever range of motion is required, the spring must be 

 capable of working through the same range ; consequently the 

 spring will be much stronger in action at one end of the range than 

 the other, unless it be made very long indeed, in which case its 

 action is uncertain and unpleasant. 



To remedy these defects the author has devised the following, 

 which possesses the advantages of both : — 



ABCD (fig. 1) is a portion of the arms attached to telescope, or 

 cradle, on which is planted the block (b), forming the bearing of the 

 screw. The nut (n) is in the form of a ball working in a socket on 

 the extremity of the clamp-arm EFGr. A short stiff spring (S) is 

 attached to this clamp-arm, bearing, not directly against any part 

 of other arm, but against end of a second screw of same pitch as 

 the main screw, the nut of which (oo) is toothed on edge, and works 

 into a wheel of equal size (pp) on main screw. The point of this 

 second screw, therefore, advances as much in one direction as the 

 frame ABCD is carried in other, according as the milled head 



