256 Scientific Proceedings, Royal Dublin Society. 
Let us inquire into any possible disadvantages that may be 
urged against this form of mounting:— 
Ist. That the temperature of the water will often be different 
from that of the air; and consequently that there will be a detri- 
mental 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, hermeti- 
cally 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. 
2nd. 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. 
drd. It may be urged that the friction of the water will prevent 
the rapid setting of the instrument. 
In a telescope of this size all the motions would be effected 
by motors of some description, guided by the observer from a 
commutator-board at the eye-end, and there would be no diffi- 
culty in setting the telescope quite as quickly as could be expected, 
considering its great size. 
