Forty-feet Reflecting Telescope. 353 
air, will bring on a premature decay, by lodging wet. In 
order to obtain steadiness, however, the cross timbers of the 
frame, in all places where they are bolted together, are let in, 
and receive each other about ^ of an inch, which makes an 
entering of i\ inch into each other, and produces the required 
firmness without any material weakening of the timber. 
The twelve rollers whose place has been pointed out, would 
not have been sufficient to support the length of the beams to 
which they are fastened, the shortest of which, as we have 
seen, being near 40 feet long. Eight additional Tollers, there- 
fore, sustain the ground timber half way towards the centre 
at LMNOPQRS. They are, like the former, directed to 
the pivot upon which this frame moves, and rest upon the in- 
ner foundation wall. 
In the centre is a large post of oak, framed together with 
braces under ground, and walled fast with brick-work so as to 
make it steady. The two central beams B B, II, cross each 
other over this post ; and a strong iron pin, or pivot, goes 
through them both into a socket within the centre of the 
post, s6 as to permit the whole of the foundation timber to 
turn freely upon this centre, when a proper force is applied for 
that purpose. 
Although by means of the 20 rollers, and this support in the 
centre, the bo tom frame of the stand to the telescope be 
firmly supported, it may notwithstanding be easily seen that 
there was occasion for some additional braces, in order to keep 
each beam in its proper situation. Eor this reason 8 pieces, 
a a, bb, cc, dd, ee,ff, gg, h h, of a proper length, 4 inches 
broad and 6 deep, are applied near the end of the beams 
against the sides of them ; these are held together by irons 
