G-rubb — Floating Refracting Telescope. 135 



its lower end into a sphere ; the construction and distribution of 

 the weights being such that the centre of gravity of those portions 

 of the instrument carried on the declination axis, that is to say, 

 the tube, object-glass, mirror, and inner sphere, shall precisely 

 coincide with the centre of the sphere. 



If this be correctly carried out, such a body will, of course, 

 float in water in a state of perfect equilibrium at any angle and in 

 any position. 



The polar axis consists of a slightly larger sphere, supplied with 

 bearings placed or mounted at the proper angle, in which the 

 telescope with its inner sphere is placed ; and sufficient water is 

 supplied to the space between the two spheres to almost float the 

 telescope with its inner sphere in the outer sphere. 



The outer sphere, in which the distribution of parts is likewise 

 such that the centre of gravity corresponds with the centre of the 

 sphere, is itself floated in a third outer sphere or portion of a sphere, 

 which is supplied with sufficient water to float the whole moving 

 parts, including the telescope, with its inner sphere and polar- 

 axis sphere. The result of the whole is that all parts are in 

 perfect equilibrium no matter what positions they are placed in, so 

 that the flotation principle in this case is carried out in the most 

 effective manner ; and any portion of the weight of the instrument 

 that is desired, from none up to the whole weight, can be relieved 

 off the bearings either of the polar axis or the declination axis, 

 and still leave all parts in a perfect state of equilibrium. 



It will be noted in the design I propose, that the driving 

 arrangements consist of practically an independent equatorial 

 instrument, the polar axis of which is placed in line with the 

 polar axis of the equatorial proper, which carries the telescope, 

 and only connected thereto by a simple driving arrangement, so 

 that any strains of flexure which may possibly occur in the 

 ponderous masses of the equatorial can in no way affect the 

 smaller equatorial, which might be called the driving equatorial. 



The advantages to be derived by the adoption of this flotation 

 principle for such an equatorial are : — 



1. That no matter what the weights of the moving parts are, 

 there should be no difficulty in dealing with them, as any per- 

 centage of the weight can be retained by water-pressure if all 

 details be well designed, and such an instrument should work 



M2 



