10 D. Todd — ( >i>< n- A ir Telescope. 



But all these designs would offer serious difficulties in actual 

 construction and operation. (1) To make a circular track 

 so near a perfect plane that the load would he eventy dis- 

 trihuted among the rollers. In experience with the 35-foot 

 dome of Amherst College Observatory, supported on 11 rollers 

 with an average weight of a ton on each, I have found equable 

 distribution of the load by no means easy, and oftentimes one 

 or two rollers travel part way round as "idlers." With the 

 number of rollers increased to 50, as in the proposed telescope, 

 I think the obstacles to equable load would be very greatly 

 enhanced. (2) To preclude overturning of the entire structure 

 by wind-thrust, a track on the upper side of the base plate 

 would be necessary; and trued up in a plane absolutely parallel 

 to that on its underside. (3) The wind-thrust on the central 

 pintle might prove so great as to deform it, and thus enforce 

 expensive and time-consuming repairs. (1) It is absolutely 

 necessary that the whole structure, weighing perhaps 50 tons, 

 shall turn with perfect smoothness and unvarying resistance, so 

 that the clock-motion in azimuth may drive it with a positive 

 motion, and with that uniformity of speed-variation requisite 

 to maintain the telescope's collimation-line always coincident 

 with the star's vertical circle. 



Temperature-adjustment of the base-plate offers no serious 

 difficulty, as the entire plate is readily enclosed within a 

 weather-proof house, in the same manner as the dome at 

 Amherst is sealed against the smoke, dust, fog and temperature- 

 flnctuations of the outer air ; that is, by means of a thick hair- 

 felt diaphragm which slips round on a stationary ring faced 

 with planished copper. Also by mounting the vertical towers 

 symmetrically on the base-plate, the stresses in it become very 

 favorable for preventing its deformation. 



All the above mentioned Hess-Bright designs contemplate 

 the sustentation of the entire load upon the rollers. As this 

 load need not exceed a ton on each, there is no reason whatever 

 to fear that the whole structure might not turn round with 

 ease, and with some approach to entire smoothness. I am, 

 however, doubtful about its being the best method, even if the 

 principle of flotation were added. 



The cardinal objection to this whole plan, which Herschel 

 was the first to employ, lies in the circular horizontal track. In 

 practical use, it would always be accumulating dust, and any 

 defect of precise radial adjustment of the axes of the rollers 

 would give rise to variable friction. This is wholly unal- 

 lowable, unless the clock-motion is a motor with excessive 

 reserve power. 



Let us see if we cannot get along without the horizontal 

 track entirely. What we want is an absolutely smooth, positive 



