G-RUBB — Machinery for the Domes of Observatories. 489 



the wall-plate, and turning the wheels to the proper taper, there is 

 no slippage necessary in order that the rollers may roll round in 

 their proper curve, while there is ample base between the outer and 

 inner wheels to ensure the steadiness of the dome. All the lateral 

 pressure is borne by the lateral rollers. The only pressure on 

 the flanges of the inner wheel is that due to any little want of 

 perfection of the mechanism, or dirt on rails, and must be very 

 small indeed. This is acknowledged to be by far the best arrange- 

 ment of revolving gear for domes, and has been adopted in most 

 of the large examples erected of late years, even in the Washington 

 and Lick Domes in America. 



In the former case, however, they omitted to plane the wall- 

 plates, and, consequently, did not get quite as good results as in 

 other cases; and in the case of the Lick Dome a considerable 

 amount of complication was introduced, the value of which is 

 doubtful. 



While the undoubted advantages of this live roller system over 

 others is freely acknowledged for large domes, and shown by the 

 fact that it has been nearly always adopted for such, there are some 

 who still appear to doubt its advantages, even in the case of 

 moderate -sized domes. The reason of this is probably due to the 

 fact that most small domes on the live roller principle have been 

 constructed on one or other of the less efficient forms mentioned 

 above, but if comparison be made between the best form of each, 

 there can be little doubt of the great superiority of the live roller 

 system. 



It can never be expected that the rollers of a dome, which are 

 compelled to travel on a circular railway, can run with quite the 

 same freedom as if running on straight rails. 10 lbs. to the ton 

 is considered a small tractive force on a straight rail. On a 

 curved rail, under the best conditions, it is rarely less than 14 lbs. 

 With the live roller system I have frequently found it to be in 

 practice under 4 lbs. per ton, sometimes as low as 3'5 lbs. per ton. 

 This is under the best conditions. Under any other conditions the 

 live rollers must have still further the advantage, for here there is 

 no oiling, because there are no bearings to oil. All that is required 

 in the case of the live roller is to keep the tracks swept free from 

 any coarse dirt. 



There is only one disadvantage that I know of about this 



