462 Todd — Untried Form of Mounting for a Telescope. 



engaging the outer surface of the sphere. Three wheels for 

 each coordinate, mounted in proper planes, all press against 

 the outside of the sphere simultaneously, and electric motors 

 then turn them at the speed required. The sphere can, of 

 course, be revolved in but one coordinate at a time; and the 

 cable connection enables the observer inside the platform 

 to throw into gear any treble set of wheels he may desire. 



Above the platform, or alongside it, are the automatic 

 setting devices, one for the horizon system, and another for 

 the equinoctial system of coordinates. At the zenith of each 

 of these is an index, which shows, as a vernier on graduated 

 circles, the pointing of the tube. These setting-systems are 

 gimbal-mounted and pendulum-controlled, the antique arm- 

 illary sphere furnishing the basis of their design. 



A peculiar sort of finder is essential — with a duplex coude 

 tube, so that its eyepiece can be brought close to the eyepiece 

 of the great tube. 



Slight differences of poise in the sphere and its appur- 

 tenances are compensated through the instrumentality of radial 

 rods, upon which travel masses of a few hundred weight, by 

 means of rack-and-pinion motors. Thus the sphere's center of 

 gravity is easily adjusted to exact coincidence with its center 

 of figure. 



Probably the moving sphere and all its accessories will 

 weigh not far from 2000 tons, and it must be as susceptible of 

 delicate adjustment and subsequent following-motion as is a 

 telescope of the usual German or English type of mounting. 

 Quite obviously such a great telescope will require clockwork 

 of exceptional power. 



Two or three methods of clock-motion are feasible : I will 

 outline but one. A huge polar axis (fig. 3) is built into the 

 exterior masonry due south of the sphere, and its upper end 

 terminates in four arms, each of which carries a large shallow 

 cup, of the exact spherical concave. These cups are faced on 

 the inside with rubber belting ; and when the chuck is in 

 action — (telescope clamped in right ascension) — they push with 

 a few hundred pounds stress against the sphere. Two rubber- 

 faced wheels, on the northwest and northeast sides of the 

 sphere, oppose the poleward thrust of the chuck. They are 

 of course mounted parallel to the equatorial plane. We 

 depend upon the polar axis and opposing wheels to preserve 

 the axis of rotation of the sphere invariable. This axis passes 

 in a constant direction through the center of figure of the 

 sphere. We apply the clock power, not to the polar axis 

 itself, but to an opposing pair of rubber-faced wheels, whose 

 planes are coincident with the sphere's equator. There is thus 

 the distinct advantage of a driving wheel 100 feet in diameter. 



