136 Scientific Proceedings, Royal Dublin Society. 



with the ease and accuracy of an instrument of one-tenth the 

 weight. 



2. The dimensions of the whole instrument, the building to 

 accommodate it, and dome to cover it, are all reduced in a very 

 large proportion, and the cost decreased accordingly. 



3. On account of the short radius which the eye-piece describes 

 as compared with other forms of telescope, the difficulties con- 

 nected with the status of the observer — that is, the means for the 

 observer conveniently to reach the eye-piece — are very much 

 reduced. 



As regards this last point, it will be seen that a pair of 

 convenient staircases are arranged, one on each side of the tube, 

 carried upon rails laid on the upper floor. This upper floor forms 

 a part of the dome, and revolves with it. 



The telescope tube is supplied with attachments for eye-pieces 

 in four places; and convenient access to one or other of these 

 eye-pieces can always be had from some part of these travelling 

 stair- cases. 



For comparison's sake, a design of the same-sized telescope, 

 drawn to a smaller scale (Plate IX., fig. 3), is given of the ordinary 

 or German form, complete with hydraulic floor, &c, which would 

 be almost indispensable in this case ; and Plate IX., figs. 1 & 2, 

 gives an outline of the buildings and dome required for the 

 ordinary and the flotation instruments side by side, and to the 

 same scale. 



No hydraulic floor is necessary in the case of the flotation 

 telescope, as the difference in the height of the eye-piece for 

 various altitudes of stars never exceeds 18 feet, while in the 

 ordinary form it amounts to about 40 feet. 



For convenience of painting, oiling, &c, arrangements are 

 provided by which an additional quantity of water can, if desired, 

 be added either to the outside trough or between the two spheres, 

 thus lifting, in the one case, the whole instrument, including the 

 polar axis, out of its bearings ; and in the second case lifting the 

 declination axis and telescope out of its bearings, thus com- 

 pletely avoiding any necessity for employing cranes or other 

 tackle for such purposes. 



It should be borne in mind that this instrument has been 

 designed for a latitude where frost is never likely to occur to an 



