460 Todd — Untried Form of Mounting for a Telescope. 



We float this sphere, in such manner as to admit the utmost 

 ease of motion in any required direction. Masonry of brick 

 or stone is built, about 25 feet in depth, and perhaps 100 to 

 120 feet square. This is constructed in the shape of a zonal 

 basin with Portland cement, and its interior is smoothed up to 

 exact figure, by erecting a platform above the center of the 

 basin, and swinging from it a convex trowel attached to a 

 radial arm. 



The sphere is of course assembled and erected piecemeal in 

 situ. A small amount of wood-preserving liquid is added to 

 the basin, as the sphere is built up ; and this suffices. to keep it 

 always just floated, thus making it very easy to turn into any 

 desired position for attaching one part after another. When 

 the steel coating of the sphere is finished, the wooden jacket is 

 next put on, the structure all the while floating in preservative, 

 till this exterior of oak is complete and trued up to figure. 

 Then the bright metal sheathing begins. When finished, the 

 preservative is withdrawn, and water substituted therefor. 

 Only a few barrels of water poured into the basin will be 

 required to float the sphere and all its subsequent appurte- 

 nances. The water is made part of a circulatory system, and 

 warmed in winter to prevent freezing. 



If we are content with a telescope a little short of 100 feet 

 in focal length, the object glass may be set in the surface of 

 the sphere. On its opposite side, the eyepieces, spectroscopes, 

 photographic cameras, and other accessories are capable of per- 

 manent and rigid attachment to its internal rib-work. 



The observer is there, too ; but free to move about on a 

 swinging platform, a good model of which is furnished by the 

 glass crystal of an ordinary ship's chronometer. The platform 

 maintains itself always horizontal, no matter in what direction 

 the axis of the telescope is pointed. 



But a sphere of 100 feet diameter very readily admits the 

 possibility of a telescope approaching 200 feet in length, if we 

 attach a tube 100 feet long to the exterior of the sphere. This 

 method of mounting the objective is illustrated schematically 

 in the engraving (fig. 1). 



At intervals of perhaps 30 feet, girdling rings are attached 

 to this exterior tube, and to these are fastened guy-rods which 

 anchor in the outside of the sphere, thereby holding the outer 

 end of the tube rigidly in position, even if the weight of the 

 objective in its cell should be as much as a ton.* 



*The author is not unaware of certain objections to the design proposed, 

 and others may be suggested. The effect of temperature inequality is one. 

 An obvious objection arises from the vibration which it is natural to sup- 

 pose a variable wind pressure would induce. But the open-air telescopes 

 of Sir William Herschel, Lord Rosse, and Lassell were subject to this diffi- 

 culty also. Experiment with a model of perhaps one-quarter or one-third 

 the full size seems to the author the best method of testing the validity 

 of these objections. 



