100 



KNOWLEDGE 



[May 1, 1894. 



have an equatorially-mounted telescope of any size, without 

 any weight whatever on the bearings of the dechnation 

 axis ; or, the tube may be hghteued by an amount nearly 

 equal to the weight of the polar axis, and there will then 

 be practically no weight whatever on the bearings of that 

 axis. So here we have a case of, say, an eighty ton 

 telescope mounted and carried by an equatorial, but 

 without throwing any weight whatever on the equatorial 

 mounting : and the force necessary to drive the instrument 

 IS then independent of the weight of the telescope, and 

 dependent only on the friction necessary to be overcome in 

 carrying the tube at an exceedingly slow rate through the 

 water. 



The spherical protuberance of the tube being fixed near 

 the upper portion, and the water-line being made coincident 

 with the centre of the sphere, which is the centre of 

 motion, it follows that the eye end of the telescope can be 

 brought to within about fifteen feet of the centre of motion, 

 and the movement of the observer need never be more 

 than three feet per hour. To approach the eye end I 

 would arrange a stage independent of the telescope, and 

 mounted on rails round the tank shown in Fig. 2. Two 

 flights of steps would enable the observer to be always 

 within reach of the eye end. 



To overcome the inconvenience to the observer due to 

 the rotation of the tube as the telescope moves in right 

 ascension I would perforate the circumference of the 

 tube at intervals of thirty degrees, attaching adapters for 

 eye-pieces at each place, and arrange the "flat-mount " in 

 a collar, so that the flat could be turned through angles 

 of thirty degrees, and the image of the celestial object to 



be a detrimental mixture at the mouth of the tube, of air 

 from inside the tube (which will partake of the temperature 

 of the water) and the cooler or warmer outside air. 



This I would propose to avoid by making the tube 

 double, with a space of some three inches between the 

 inside and outside tubes, hermetically closed except at 

 the lower end, where there would be apertures in the inside 

 envelope. 



The space between the two tubes would be connected 

 through the trunnions with an air pump, worked by a gas 

 or other motor, which would continually exhaust the air 

 from between the two tubes, and thus cause a current of 

 the outside air to pass continually down the inner tube 

 and to the pump through the space between the two 

 tubes. This would keep the temperature of the inside 

 tube and the air in the tube constant with that of the 

 outside air. 



A velocity of motion of the air of one foot per second 

 would perhaps be quite enough to entirely overcome this 

 difBculty. 



To avoid dewing of the mirror when the air outside was 

 warmer than the temperature of the water, I would 

 propose to warm the mirror by electric arrangements at 

 its back, always keeping the mirror one degree above the 

 temperature of the air. 



Second — The limited range of the equatorial. I have 

 stated that the instrument would be in perfect balance 

 down to twenty-five degrees from the horizon. If desired, 

 though no longer perfectly balanced, it can be used lower 

 by employing a chain or wire rope connected between the 

 lower end of the tube and the upper end of the polar axis, 



Fio. 2. — Section of the whole lostrumeut, 



showing the tube, polar axis and tank, fogethpv with the motors for producing 

 the necessarv motions. 



be observed would be thrown through the perforation of 

 the tube which was most convenient for observation. 



Let us inquire into any possible disadvantages that may 

 be urged against this form of mounting : — 



First — That the temperature of the water will often be 

 different from that of the air, and consequently there will 



and the amount which the instrument would be out of 

 balance, between twenty-five and twenty degrees, would be 

 very trifling. 



Again, it will not be convenient to use the instrument 

 within some fifteen degrees of the pole. It could be 

 planned to go somewhat closer, but when it is considered 



