Grubb — improvemenis in Equatorial Telescope Mountings. 225 



be inoperative, as the bar X would then hang vertically ; nor, as we have 

 seen before, is it required. In every other latitude, however, it will be 

 seen that the heavy weight W acting on the lower end of the lever X, 

 pivoted in plate e <?, exerts a force on the axis exactly counterbalancing 

 the component B C in fig. 3 ; and, furthermore, that this apparatus, though 

 attached to and carried by the polar axis, exerts a force whose direction 

 is constant as regards gravity, but variable as regards the polar axis, &c. 

 When the instrument is to the east of the pier, one set of rollers acts ; when 

 to the west, the other set — whichever happens to be below ; and in a position 

 six hours off the meridian, the pair of rollers Y act and take off the end 

 pressure of the axis. 



Having disposed of the component B C, fig. 3, Plate XVIII., we are 

 prepared to deal with the other component A C. This is easily managed, for 

 its direction is constant as regards the polar axis and cross-head. At b, b, b', b', 

 figs. 4, 5, and 6, Plate XVIIL, are placed a pair of bracelets (so-called), i.e., 

 frames carrying rollers, and these are connected together by two side levers 

 centred at H at each side of the centre of cross-head, the amount of force 

 being regulated through the nut h, by screwing up which the whole declina- 

 tion axis, telescope, counterpoises, &e., can be lifted out of the 1^ bearings, 

 and the weight of all transferred to the fulcrum H. In practice, as with other 

 bearings, a sufficient portion only of the weight is lifted, and the remainder is 

 allowed to rest on the Y bearings. 



The application of these "bracelets" would not be possible until the 

 internal arrangements mentioned above are first brought into action. 



The system above described is good in principle, and has been applied with 

 success to many equatorial mountings; but practical difficulties have arisen in 

 applying it to the newer, and, more particularly, to the larger, forms of 

 equatorial mountings, one of the principal being the difficulty of getting a 

 sufficient amount of weight on the internal lever. This weight being neces- 

 sarily inside the polar axis is limited in its size, and, moreover, there is no 

 possibility of getting the fulcrum high enough to obtain a fair leverage, as 

 the declination axis itself comes in the way. 



Various modified forms of this anti-friction arrangement have been tried. 

 I shall confine myself to a description of the latest form, which I have adopted 

 for all the later instruments of large size, as it leaves little to be desired. 



In this form, figs. 1, 2, and 3, Plate XIX., the centre of gravity of all 

 parts that turn with the declination axis (i.e., the telescope itself, cradle, 

 declination axis, &c.) falls very near where the larger anti-friction ring or 

 bracelet surrounds the axis, so, there being very little weight on the smaller 

 bearing at the other end, and the axis here being small in diameter, there 



2 M 2 



