D. Todd — Open- Air Telescope. 7 



rising-floor, when added to the cost of the instrument itself, 

 practically prohibit much farther increase of size.* 



Solution of the problem of the long telescope, then, resolves 

 itself into two general divisions : (1) to build the tube so that 

 its flexure shall be negligible. The elder Herschel and Lord 

 Rosse both succeeded in this ; but their tubes, although the 

 largest ever built, were relatively short. (2) to mount the 

 weighty tube so that it can be pointed with ease to any part of 

 the sky. The mechanical genius of Herschel selected the only 

 feasible type of mounting for a structure of such great weight ; 

 and with certain modifications embodying the engineering and 

 mechanical practice of the present day, his design must remain 

 forever the basis of that type of mounting, which, all told, is 

 the best, for future great telescopes. Lord Rosse, however, 

 adopted, not this form of universal mounting, but a modified 

 type in which his " Leviathan " was restricted in use to small 

 hour-angles between two north and south walls. 



I have begun my projected telescope not at the end but in 

 the middle, just as a bridge engineer builds a cantilever. 

 (Fig. 6.) 



The basis of its tube is a cubical section or compartment of 

 steel plates, reinforced as in box-girder construction, so as to be 

 absolutely rigid and unyielding. For a telescope 200 feet or 

 300 feet long, I would build this central cube about 20 feet 

 square. On two opposite sides it is perforated to allow the 

 cone of rays to pass through ; and on two other sides of the 

 cube, at right angles to the cone of rays, are attached the cir- 

 cular bed-plates of the bearing-pins. In other words, tube and 

 axis are the same in construction as the ordinary type of transit 

 instrument, with shortened axis and a minimum distance be- 

 tween the pivots. Needless to say, this is the form which, in 

 the evolution of transit and meridian circle, has been found to 

 give minimum flexure. 



As we are per force restricted to standard commercial forms 

 of structural steel, the two halves of the tube must be built 

 np, not as cones (the ideal .form) but as square pyramids. As 

 we have rotation about only one axis to deal with, the flexure 

 of the great tube is easy to handle. 



Mr. Spencer Miller, the eminent cable engineer, suggests the 

 construction of a 10-foot model of this tube, from standard 

 brass angles. Subjecting this model to known stresses and 

 measuring up the observed flexures would afford the data nec- 

 essary in deciding what forms and weights of structural steel 

 would be best adapted to the construction of the full-sized 



* All the previous illustrations are taken from my Stars and Telescopes, by 

 courtesy of the publishers, Messrs. Little, Brown & Company, Boston. 



