1894.] on the Astronomical Telescope. 317 



sections along the axis A B can be so distributed that the tube will 

 equally well remain in any other position, except it be so far turned 

 over that the cylindrical part of the tube is lifted out of the water at 

 one end and dipped at the other. 



By making the spherical part of about the proportions of the 

 figure, the tube can be depressed to within 25° of the horizon, and 

 still remain in perfect equilibrium. 



Now, suppose the tube to have a pair of trunnions attached at the 

 water line, and these carried on a polar axis of, say, the English type 

 (see Fig. 2), we have an equatorially-mounted telescope of any size, 

 without any weight whatever on the bearings of the Dec axis, or, the 

 tube may be lightened 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 80- 

 ton telescope mounted and carried by an equatorial, but without 

 throwing any weight whatever on that equatorial ; and the force 

 necessary to drive the instrument is independent of the weight of the 

 telescope, and dependent only on the friction necessary to be over- 

 come in carrying the tube at an exceedingly slow rate through the 

 water. 



Let us inquire into any possible disadvantages that may be urged 

 against this form of mounting : — 



1. That the temperature of the water will often be different from 

 that of the air ; and consequently that there will 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, with the outside air. 

 This I would propose to avoid by making the tube double, with a 

 space of some 3 inches between 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 tube and back to the pump by 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. 



2. The limited range of the equatorial. I have stated that the 

 instrument would be in perfect balance down to 25° 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, and the amount which 

 the instrument would be out of balance, between 25° and 20°, would 

 be very trifling. 



Again, it will not be convenient to use the instrument within 

 some 15° of the pole. It could be planned to go somewhat closer, 

 but when it is considered that nine-tenths of the work required to be 

 done can be commanded by this instrument, it is clearly better to 

 design it to do that nine-tenths well than to strain it into doing 

 another 5° that would only be useful on very rare occasions. 



