700 EAEL OF KOSSE ON THE CONSTEIJCTION OF SPECULA OF 6-FEET APEETUEE, 
action of the counterpoises, which move in an arc of a circle instead of the proper curve. 
A slow hand motion was originally fixed near the mouth of the tube, for raising or 
lowering it in taking measures ; but we do not find it necessary. The telescope at the 
equator can conveniently follow an object three-quarters of an hour; and its motion is 
nearly equatorial : it would be almost exactly so if the pulley of the suspending-chain 
was in a line drawn from the axis of the universal joint parallel with the axis of the 
earth. The pulley, to give the. chain more mechanical advantage in raising the telescope 
when very low, was placed above this line ; but there was at the same time an arrange- 
ment made so that the chain might be brought, when necessary, by means of a grinding 
pulley, into the proper line. In practice we have found the movement of the telescope 
sutficiently equatorial without this : at a little distance, however, from the meridian, the 
plane of the position-circle of the micrometer deviates sensibly from a plane passing 
through the pole in all positions except when near the equator, as will be evident on 
considering the construction of the universal joint; and the distance from the meridian 
must be known where much precision is required in the reduction of the observations. 
The motion in right ascension is by a rack the extremity of which bears by rollers on 
a circular arc of 40-feet radius. This rack is connected with the tube by a pinion, and 
the pinion is acted upon by an endless screw driven by a pulley, which pulley is driven 
by a band from a porter’s wheel attached to the lower end of the tube. The pulley can 
also be moved by the observer ; but this is not often necessary. The large circular arc 
is in pieces 5 feet long each, carefully planed, but not touching at their extremities, to 
guard against unequal action. The surface of each segment was adjusted separately to 
the plane of the meridian by a transit-instrument. And thus the means were provided for 
taking right ascensions with considerable accuracy. For polar distances there is a circle, 
18-inches radius, at the lower end of the tube, furnished with a spirit-level ; but for find- 
ing objects, there is an index of 6 -feet radius connected with the transverse axis of the 
universal joint; so that the instrument can with the utmost facility be set roughly in 
polar distance. Means, of course, are provided for enabling the observer to reach the 
eyepiece in every position of the telescope. From 120° of polar distance to 80° this is 
effected by a stage, nearly counterpoised, which slides on bearers, the observer standing 
in a small gallery, to which he can communicate a transverse motion : the large stage is 
raised by a windlass ; and, to guard against possible accident, there is an arrangement 
which locks it, completely under the command of the observer. From 80° to 50° the 
eyepiece is reached from the second gallery, and from 60° to 25° by the third. The 
fourth gallery, reaching to the pole, for which machinery was made at the same time 
as for the others, has never been put up, the other galleries so far furnishing ample 
employment for the instrument. The action of the galleries, and the way they are 
secured, is sufiiciently evident from the photograph : they are of great strength, and in 
their construction, as well as in all other parts of the instrument from which there 
seemed to be a possibility of accident, the ordinary engineering rules as to strength have 
been considerably exceeded. The eyepiece arrangement consists of two adapters fixed 
