On the Cork Equatorial Telescope, &c. 359 
The Dome is of fifteen feet diameter, constructed of wrought 
iron ribs and purlins, covered with— 
Ist. A network of wire, interlaced. 
2nd. A thickness of sailcloth or canvas. 
ord. A layer of felting ; and 
4th. A second covering of sailcloth or canvas, well painted. 
The Dome (Fig. 8.) has a cast-iron sole plate, planed truly 
flat on the lower side, and the wall is capped with a cast-iron 
wall plate, having ribs cast in it, and accurately planed, and 
between the wall plate and sole plate roll eight rollers carefully 
turned to a conical form. ‘This set of wheels or rollers, coupled 
together by a wrought-iron framework, constitute what is tech- 
nically called a live ving; and as all the friction is of that 
character called rolling friction, and not sliding, the tractive 
force necessary to drive the dome round is extremely small. 
Lateral rollers are applied to prevent the Dome getting out of 
centre. One important feature of this system of mounting 
revolving Domes is, that as there are no bearings or pivots 
there is nothing to clog or get stiff, and the Dome works as well 
(if only kept moderately clean) after ten years as when first 
erected. The Great Domes for Vienna Observatory I mounted on 
this principle. Three of these were of 27 feet diameter, and one 
of 45, v.¢., the largest revolving Dome in existence ; and even this 
Dome, that weighed sixteen tons, required only a traction force of 
70 lbs, to drive it without any gearing, 7.¢.,st¢ of its own weight. 
