Royal Astronomical Society, 5SS 



less than 50 feet above them. But for the larger mirror such a trial 

 is impracticable (the height of the object must exceed 100 feet), 

 and the mirrors have therefore been placed in the telescope without 

 any trial, and their definition has been found perfect. 



Mr. Lassell uses for polisher a wooden plate formed of two thick- 

 nesses of pine- wood with the grain crossed ; and this apparently 

 yields to accommodate itself to the form of the mirror. In the arrange- 

 ment of its square protuberances, it is similar to Lord Rosse's ; but 

 it is covered with only one coating of pitch. The polishing powder 

 used is the same as Lord Kosse's. The Astronomer Royal believed 

 that the attentions to temperature, moisture, &c., which Lord Rosse 

 found indispensable for his large mirrors, are not found necessary by 

 Mr. Lassell. Mr. Lassell finishes the operation with the speculum wet. 



VI. The next point to which allusion was made is the form and 

 mounting of the telescopes. 



Lord Rosse's telescope js a wooden tube, its interior diameter 

 exceeding 6 feet in every part, being at the middle about 7 feet, and 

 nearly 50 feet in length. This is fixed to a cube of 10 feet, which 

 has folding-doors on that side which, when the telescope is hori- 

 zontal, is the upper side (at which side the fixed frame supporting 

 the mirror is introduced, as has already been said), and which carries 

 the fixed frame by three large screws in that side of the cube which 

 is opposite the mouth of the telescope. To this side of the cube is 

 attached the universal joint by which the lower end of the telescope 

 is connected with a fixed support, the joint being a few feet below 

 the general surface of the ground. On each side (east and west) 

 of the telescope is an enormous pier of solid masonry, about 70 feet 

 long, in the north and south direction, between 40 and 50 feet high, 

 and in its thickest part nearly 20 feet thick. [None of these dimen- 

 sions are taken from actual measure.] The fixed support is nearer 

 to the north than to the south ends of these piers. Near the top of 

 the piers, on the interior faces, in the east and west plane passing 

 through the universal joint, are two cranes with pulleys (the turning 

 crane being no bigger than suffices to carry a large pulley, whose 

 edge is in the vertical axis of the crane) ; over these cranes the 

 chains pass which are attached to the telescope ; and to the lower 

 ends of the chains, after they have passed fixed pulleys on the walls, 

 are attached the counterpoises, weighing about four tons each. 

 These counterpoises are not allowed to depend freely, but are con- 

 nected by bridle-chains with wooden horns that project from the 

 north ends of the piers ; the effect of this arrangement is, that when 

 the telescope tube is nearly horizontal, and the force required to 

 support it is very great, the weight of the counterpoises acts very 

 nearly vertically on the chains, and is entirely effective for the sup.- 

 port of the telescope ; but when the telescope is considerably ele- 

 vated, and less supporting force is required, the weight of the coun- 

 terpoises is supported in a great measure by the bridle-chains, and 

 very little tension is given to the supporting chains. For the sake 

 of supplying some slight defects in the laws of tension thus produced, 

 and also for the sake of constantly producing a small tendency in the 

 telescope towards the south horizon, other counterpoises, in a pit 



