486 Scientific Proceedings, Royal Dublin Society. 



the roller. Any lateral pressure is borne by the special lateral 

 rollers, which revolve on a vertical stud attached to a bracket on 

 the dome. This roller should not be in very close contact with 

 the wall-plate, so as to allow, as before, of a little alteration of 

 form of dome without danger of locking. 



While speaking of lateral rollers, it may be observed that the 

 practice of attaching the roller to a stud on the wall-plate to 

 revolve against the dome, as shown in fig. 6, instead of as shown 

 in fig. 2, is bad. If the dome go out of shape, the roller in fig. 2 

 can be adjusted, and will still have a true ring to revolve against, 

 but in fig. 6 no adjustment of roller will cause the ring against 

 which it rolls to touch the roller all round if it be not true in 

 itself. 



Fig. 3 is a modification of fig. 2, in which the roller is fixed in 

 bearings on the wall. This is good in small domes, but necessitates 

 that the rail on the dome which rolls on roller be very true. 

 Either figs. 2 or 3 would make excellent designs for small domes 

 if the bearings of rollers were carried on ball bearings, as in most 

 modern bicycles. 



In all the preceding forms it is necessary that the bearing 

 surfaces of wheels be made very narrow, or there will be a 

 considerable decrease of efficiency due to the fact that the outside 

 edge of the wheel has a greater journey to perform than the inner. 

 If unrestrained, such wheels would tend to roll on in a straight 

 line, and consequently some power is lost in forcing them to roll 

 round in the curve of the rail. To obviate this, taper wheels are 

 sometimes made, as in fig. 4, in which case the wheel is so much 

 taper, and has its axis so much inclined, that lines drawn as a 

 continuation of the edges and of the axis would meet in the centre 

 of the dome, and on a level corresponding to the top face of wall- 

 plate. Made in this way there is no slippage necessary even in a 

 wide roller, and consequently no loss of power from this cause. 



We now come to a totally different system, viz. that commonly 

 called the live roller system. The simplest form of this is shown 

 in the figs. 5, 5a, and bb. It consists of a set of spheres or cannon 

 balls I'evolving in a grooved wall-plate, on which balls the dome 

 itself revolves; the great advantage of this system being that 

 there are no bearings, and therefore no sliding friction, and that 

 no oiling is required. 



