156 
THE EARL OF ROSSE ON SOME RECENT IMPROVEMENTS MADE 
In modifying this mounting we endeavoured to make the polar axis as much a con¬ 
tinuous girder as the conditions to he fulfilled admitted of. The upper bearing was 
placed externally to the structure and was reduced to a small diameter. Unlimited 
freedom of motion round the polar axis was thus sacrificed; but it is believed that a 
more rigid and steady mounting has been obtained which, without the introduction of 
antifriction rollers, turns with at least equal ease in Plight Ascension. The mounting 
(Plates 11, 12, and 13) is carried by a massive pier of stone laid in cement-mortar, on 
the southern inclined face of which rests a bed plate (A A, B B) of cast-iron, ribbed on 
its under side, imbedded in cement and fixed firmly down by six bolts, four of them 
near the lower and two near the upper edge of the plate, and all extending deep into 
the masonry. Upon this rests, with power of adjustment in both planes, the pedestal 
casting (C), which is bored to receive the polar axis, a wrought-iron bar with slightly 
conical bearing surfaces near its ends fitted into it. A cotter and wedge received by 
a slot through its centre press it down firmly home, and the same when reversed in 
position can be used to start it up from its place if at any time required. The 
pedestal casting is provided with a lip which catches the top edge of the bed-plate, 
keeps the former from sliding down, and takes off all side pressure from the adjusting 
screw, D. The centre of gravity of the whole overhangs the upper edge of the base¬ 
plate a little. 
The fork is tubular and made of boiler-plate j-incli thick, firmly rivetted to angle 
iron, of 2j : X 2 y X -j^-inch scantling, along each angle in the usual manner. The web 
of the lower part is carried up and connected to the base of the fork at F F to give 
greater rigidity. The fork, as will be seen from Plate 11, fig. 4, turns on conical 
bearings of small diameter. The bearings are of hard brass. Their external surfaces 
are cylindrical, and they are fitted into castings rivetted to the plating of the fork. 
The bearings can be turned round,'" and thus, the common axis of the conical cavities 
being inclined at an angle of about 1 3' from that of their external surfaces (the cavity 
in the lower one being eccentric by -yg- of an inch), an adjustment for perpendicularity 
of the declination to the polar axis can be made, and no provision for adjustment of 
the bearings of the declination axis is needed. The counterpoise of the fork is marked 
W in the drawings. 
The tube turns on cylindrical trunnions which have conical ends fitted into castings 
rivetted to the sides of the tube. They are retained securely in them places by 
feathers, nuts, and lock-nuts. The pressure of the tube parallel to the declination 
axis is communicated through the shoulder of the lowermost trunnion to the corre¬ 
sponding branch of the fork, and also equally to the other branch through the washer, 
* Dotli bearings are provided with graduations, so that they may be turned round by an exactly equal 
amount, and that thus the axis of each may continue parallel to the axis of the bar, the whole describing 
a conical surface round the common axis of the external surfaces of the bearings. The component of the 
motion in the plane of the declination axis is that which effects the adjustment, that perpendicular to that 
axis having no effect beyond a slight disturbance of equilibrium round the polar axis, 
