1894.] on the Astronomical Telescope. 315 



will be made to build an 8 or 10-foot reflector for the great exhibition 

 to be held in Paris in 1900, it may be interesting to consider the 

 conditions desirable to be fulfilled for such an instrument, and the 

 most promising construction to s-itisfy those conditions. If a 

 monster telescope, such as this, is to be mounted only in such a 

 manner as will satisfy the ordinary conditions of star gazing, I 

 fear the results will be disappointing, but let it be mounted in such 

 a manner as to render it useable for the more delicate and refined 

 work of the modern astronomy, and a grand and productive field of 

 work is open to it. 



But the problem of mounting an enormous instrument such as 

 this, whose weight would probably amount to from 50 to 100 tons, 

 so perfectly poised and so accurately driven by clockwork as never 

 to vary from its true position by a quantity greater than the 

 apparent motion of a star in one-twentieth of a second of time, is 



sufficiently difficult to justify almost a doubt of its possibility, and 

 this difficulty has been appreciated by others ; for Dr. Common, who r 

 as the maker of the largest equatorially mounted reflector ever 

 completed, must be considered as the first authority, proposed some 

 short time since to resort to the alt-azimuth form of mounting, with 

 which it would, of course, be impossible to satisfy the above condition. 

 Dr. Common himself has made a splendid advance in adopting 

 the system of flotation of the polar axis ; this principle of flotation 

 appears to me to be capable of further development. It is perfectly 

 possible to make a tube for a Newtonian reflecting telescope (which 

 is necessarily closed at the lower end) of such a weight, and with 

 its weight so distributed that it will not only float in water sub- 

 merged to a certain point (preferably near the upper end), but will 

 be in a state of equilibrium when placed at any or in every position 

 down to a certain angle, which angle depends on the exact outside 

 form of the tube. For instance, if A B (Fig. 1) be a tube closed at 

 B and perfectly symmetrical round the axis A B, and the total 

 weight of the tube be equal to the weight of water which is dis- 

 placed when the tube is sunk to C, the weight of the different 



