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XXXII. 



KEVOLVING MACHINERY FOR THE DOMES OF ASTRO- 



J^OMICAL OBSERVATORIES. By SIR HOWARD GRUBB, 



F.R.S. 



[Read May 22, 1891]. 



The convenience and ease of motion of the Observatory Dome, 

 though a secondary, is still a very important question to the 

 astronomer. 



To an engineer who has not had actual experience in the 

 construction of such domes, the problem of designing a system of 

 revolving machinery for carrying the weight of a few tons may 

 seem a very simple one, and as a consequence of this, some 

 lamentable failures have occurred in cases of roofs constructed 

 from the designs of engineers of great eminence, indeed, but 

 without experience of the special conditions required to be fulfilled. 



In designing the revolving machinery for these roofs the 

 general conditions are often assumed to be identical with that of a 

 railway turn-table for locomotives ; but there are many differences : 

 for instance, in the latter case, it is possible to have a centre on 

 which the table will revolve, which is clearly not possible in the 

 case of observatory roofs. This is a most important difference, for 

 in the first place any lateral pressure (which in the case of domes 

 or drums is sometimes very heavy from wind or other causes) is 

 borne on a central pin which is probably less than^^th part of 

 the diameter of the track of the wheels, and consequently moves 

 100 times slower ; and also because it is then possible, in the case of 

 live rollers of the form I shall presently explain, to centre the 

 framing which connects these rollers together on the central pivot, 

 and thus insure that the axes of the rollers are kept precisely 

 radial, in which case the maximum of efiiciency is attained. 



In the construction of observatory roofs experience shows that 

 due allowance must be made for alteration of form due to differences 

 in temperature and hygrometric conditions of atmosphere or from 

 variation of strains under differing conditions. It was from neglect 

 of these precautions that the failure of some of the earlier domes 



