D. Todd — Open- Air' Telescope. 11 



and yet exceedingly easy motion of a huge vertical shaft. In 

 order to meet those requisites, and treating it as a shaft simply, 

 we should extend its length downward, and confine its motion 

 as a journal between two horizontal friction rings, slipping 

 between roller bearings on vertical axes. These rings, with 

 perhaps 100 roller bearings for each, 50 outside and 50 in, 

 should have horizontal wheels or rollers of large diameter, 

 ground to perfect cylinders. These rings are intended to 

 operate only when the telescope is in use in the wind. "When 

 the air is still, a smoother azimuth motion would be possible 

 if the friction rings are clamped motionless to the concrete 

 well in which the vertical drum revolves. 



Then I would float the entire structure in either oil or water, 

 leaving only a few hundred pounds of load on a central button 

 or pintle. In this way a clock of minimum power would 

 suffice to drive a maximum load with perfect smoothness. 

 The expanded vertical axis, then, would be a little like the 

 inner tank of a gasometer when at its lowest point. It should 

 be not less than 50 feet in length, or vertical height ; and 

 this drum-shaped form would afford an easy interior construc- 

 tion insuring absolute rigidity of the vertical towers, which 

 might go down through it, as far as thought best. JSIo wind 

 could ever topple the structure over, and the open-air telescope 

 would be safely usable on all but the windiest occasions. By 

 lowering the pintle underneath, and pumping out enough of 

 the flotation to allow the drum to settle down to stationary 

 beds, the drum could be rigidly clamped near the top and 

 bottom, so as to withstand securely all stormy weathers when 

 not in use. In case of a severe gale, the telescope would be 

 pointed quartering to the wind, observing carriage and tail- 

 piece unshipped, and tube directed to the nadir. Here the 

 objective would be double-housed against the storm, and the 

 cell clamped firmly. This would secure both bearings and 

 open-air pyramids against harmful stress clue to excessive wind- 

 thrust, as the gale would strike the structure edgewise. 



I have not intended in this paper to deal with the solution 

 of any but the most general problems of this proposed tele- 

 scope. All details have, however, been critically worked out, 

 so that an approximate estimate of cost is available. A 5-foot 

 objective would cost about $125,000, and the entire instrument 

 would represent about double that amount. For instance, the 

 weather-protection of objective and eyepieces, of altitude bear- 

 ings and of altitude clamp, quick motion and slow motion ; 

 also electric motors for operating the same in both coordinates, 

 and for the requisite variable clock-motion in both altitude and 

 azimuth, as well as for driving the rotary tail-piece on a ball-race. 

 With this arrangement, and an adaptation of Prof. Ritchey's 



