Grubb — Machinery for the Domes of Observatories. 49J 



great advantage of tlie live ring of rollers is secured, that is, that 

 there is no friction but rolling friction, and also that there is no 

 danger of any unsteadiness in the motion, such as is the case in 

 form shown in fig. 6, as the base upon which the rollers stand is 

 of good breadth. At the same time it is not necessary that the 

 surfaces of the wall-plate should be planed to the troublesome 

 conical form of that required in fig. 7. Strictly speaking, the 

 upper surfaces of the wall-plate should be slightly conical, but the 

 amount of this is so small [i. e. the proportion between half the 

 diameter of the axis of the roller to that of half the diameter of 

 the dome itself) that it may with perfect safety be neglected. As 

 the working parts a,re all planed surfaces, the expense is very 

 much less than that of the form shown in fig. 7. 



The lateral rollers, to keej) the dome from sliding horizontally, 

 may be attached in any convenient position, such as is shown in 

 fig. 9, and, if desirable, that portion of the wall-plate may be 

 planed to a true circle. Each roller is supplied with a loose 

 washer at the outside in order to reduce any friction which may 

 occur between the inside edge of the groove and the shoulder of 

 the roller, when from any cause the roller may rub against the 

 wall-plate, but any friction here must be very small, because, for 

 any given motion of the dome the live ring moves, not half, as in 

 the case of fig. 7, but in a much smaller proportion, in fact only in 

 the proportion of the necessary diameter of the axis to the diameter 

 of the roller itself. 



