Gtrubb — Floating Refracting Telescope. 137 



extent sufficient to freeze the water ; but if required for higher 

 latitudes, there should be no difficulty in adding some chemical to 

 the water that would prevent this inconvenience. 



All the various motions of the instrument, setting in right 

 ascension and declination, &c, also movement of dome, are 

 arranged to be driven by motors. 



The declination readings would be taken from an arc of about 

 10 feet radius, attached to the outer sphere, alongside the opening 

 through which the telescope protrudes ; and this is quite close to 

 the eye-piece and observer. 



Arrangements for reading the right-ascension circle can also, 

 if desired, be made to enable the observer to take his readings 

 while at the eye-piece of the telescope. 



An obvious objection may be raised to this form of instrument 

 on the ground that with sudden changes the upper end of the 

 tube will probably be of a different temperature from the lower, 

 where it is immersed in the water ; but I have already provided 

 for this in my design for the reflecting instrument by making the 

 tube double, and keeping a constant circulation of air of the same 

 temperature as that surrounding the upper part of the tube 

 passing between the double envelopes and round the sphere at the 

 lower end. Similar arrangements are provided in this case. 



I may mention that, since I proposed this last arrangement 

 for the reflecting telescope, a very similar plan has been actually 

 put into practice at an American observatory with very satisfactory 

 results. 



