S6 
Sir George Shuckburgh's Account 
This little window affords a constant view of the pole star in 
its whole revolution, if the sky be clear, and consequently an 
opportunity of comparing a transit and altitude of any star to 
the southward directly with it ; and, by moving the conical 
roof round, any part of the sky may be exposed to the tele- 
scope. The rollers * s, t, roll upon a surface of lead, melted 
and cast into a circular groove in the timbers at EE, and 
planed truly horizontal. Besides the apertures already de- 
scribed in the roof, the room is enlightened by two windows 
to the south-east, and north-west; and also by two oval lights 
in the side of the cone, nearly east and west, and by a third in 
the zenith at r. v, w, is a platform on the outside of the 
cone, covered with lead ; of use in examining the roof, 
and cleansing it in time of snow, See. y and x are iron ba- 
lusters, very necessary on such occasions, with the iron rail 
at top at x placed obliquely, in a plane tending to the centre 
of the instrument, in order that the least possible light from 
any object may at any time be lost to the object glass. C is 
one of the iron bars passing through the wall of the building, 
fixed by a collar and nut on the outside, and resisting the 
pressure of the instrument, whose weight, without the sup- 
porters, is full 300 lb. against the support D. e,j, \ l, and g, b, 
n, are two of the side walls of the building, rising 40 feet 
above the ground, and serving as butments to the arch /, m, w, 
which is farther strengthened by a square frame, of heart of 
oak timber, 9 inches by 6 inches, let into the wall at 5, 6 , 
held together by iron bolts, and going round the building, 
* Besides these rollers, whose axes are horizontal, there are 3 or 4 other rollers, upon 
a vertical axis, exterior to, and bearing against the ring that constitutes the base of the 
cone ; by this means they keep the centre of its motion always in the same place. 
