A SELECTION EEOM THE OBSEEVATIONS MADE WITH THEM. 699 
quite manageable ; and a travelling crane, with railway overhead and screw, etFects the 
same thing for the 6-feet polisher. Both polishers are provided mth gimbals, so that 
they can be instantly turned over. Though it is better to prepare the polisher fresh each 
time, we have often employed the same polisher successfully two or three times. In 
that case the polisher must be washed, and when dry the surface is to be very slightly 
moistened with spirits of turpentine. A thin film saturated with rouge will thus be 
removed ; and a fiame passed under -will evaporate the turpentine. The polisher is then 
to be inverted and warmed to about 80°, the face being uppermost and again turned 
over. If now a flame is employed cautiously two or three times at intervals, the pitch 
at each square will become protuberant, bearing the hard resinous film on its surface, 
and the polisher will be restored very nearly to the same state it was in when originally 
prepared. 
No one will be so ill advised as to attempt to construct a large reflecting telescope 
without first collecting all the information to be obtained in books. In Mr. Lassel’s 
paper in the ‘ Transactions’ of the Astronomical Society he will have an excellent guide. 
Should he employ an apparatus similar to ours, the speculum is first to be truly ground 
with the jointed bar. The throw of the first eccentric is to be one-third the diameter 
of the speculum, and that of the second, measured at the speculum, about one-fourth. 
It will be better in all early experiments to rely on pitch alone, carefully adjusted to 
the temperature at which it is to be used, perhaps 55°. The jointed bar which was 
employed in grinding the speculum is to be exchanged for the rigid bar, the eccentric 
and guide being readjusted. When the speculum has been successfully polished a few 
times in this way, an attempt may be made to obtain a better result by facing the 
polisher with a hard resinous composition ; and finally the jointed bar may be resorted 
to, but at the same time the thickness of the pitch must be greatly reduced. 
As to the mounting, it is simple, and any engineer could execute it without difficulty, 
the photographs supplying the necessary information. 
The tube is supported at its lower extremity by a massive universal joint. It is 
counterpoised by weights which are constrained to move in a circular arc which nearly 
coincides with the curve of equilibrium ; and a steady strain is kept upon the suspend- 
ing-chain by means of three weights attached to levers, Avhich successively come into 
play as the tube approaches the zenith and passes north beyond it : the levers are about 
two-thirds of the length of the tube, and have cross heads at their low^er extremities, 
which are formed into bearings ; and when in their places, the cross heads are all parallel 
to each other and to the transverse axis of the universal joint, from which they are 
about 5 feet distant. The levers thus move steadily in one plane, that of the meridian. 
A chain connects the levers at the proper intervals and the tube -with them ; and as the 
tube descends, each lever takes its place successively in a deep recess in the ground, the 
chain subsiding into a heap. This contrivance is effectual, and the chain has never 
fouled. The three weights are of different sizes, so proportioned as to reduce as much as 
possible the de\iations from exact equilibrium at different altitudes, due to the irregular 
