134 Scientific Proceedings, Royal Dublin Society. 



of unusually long focus mounted as an equatorial, but the long 

 focus necessitated a very large equatorial if constructed in the 

 ordinary manner. He therefore suggested that the tube should 

 be made one-half the length of the focus of the telescope, and that 

 a plane mirror should be inserted at the lower end of the tube 

 nearly at right angles to the axis, which would reflect the 

 rays, but without altering their convergence, to an eye-piece or 

 photo-plate at the upper end of the tube situated beside the 

 object-glass. 



This renders the outward form of the refracting telescope very 

 similar to that of the Newtonian reflector, and enables us to apply 

 the same principle of mounting as I suggested before in the case 

 of the reflecting telescope. 



In the present case I had to design an instrument for a low 

 latitude, 25° ; and I found that the flotation principle could be 

 even more perfectly carried out in low latitude's than in higher 

 latitudes, such as ours. 



In the design before submitted for the reflecting telescope, the 

 flotation system, though it allowed of the bearings being relieved 

 of a very large percentage of the weight of the moving parts, was 

 not quite perfect, because, as will be seen by reference to the 

 drawings which accompany that paper, the polar axis (which 

 consists of a double-steel framework, embracing a sphere, which 

 sphere forms the declination axis) is more or less immersed 

 in the water according to the hour-angle of the object under 

 observation. 



In constructing such an equatorial for low latitudes, however, 

 as in the present design, this flotation principle can be carried 

 further ; in fact, it can be made theoretically perfect, so that, if 

 desired, the whole weight of the instrument can be relieved from 

 the bearings, and the balance made equally perfect in all possible 

 positions. It is desirable, of course, to leave a certain percentage 

 of the weight on the bearings in order to ensure perfect steadiness ; 

 but any percentage, up to, say, 95 per cent., can be relieved by 

 the water-pressure ; and this is equally true of both declination 

 and polar axes. 



In the present design (see Plates VII. and VIII.), which is 

 for a refracting telescope of 48 inches aperture, the moving parts 

 of which would weigh about sixty tons, the tube is enlarged near 



