Forty-feet Reflecting Telescope. 371 
part, great strength is requisite to support it firmly, as also an 
extensive connection of this strong part with the length of the 
tube. The speculum likewise is to be put in here, and when 
the telescope is in use the cover of the speculum is to be taken 
off, and afterwards to be put on again ; for which reason a 
convenient door or opening must be had. The line of collima- 
tion of the mirror also requires an adjustment at this end of 
the tube ; and a small side motion is required upon the pivot 
of the axle, which must not only be perfectly smooth, but 
equally firm and steady. All these exigencies have been pro- 
vided for in the following manner. 
Fig, 34. represents the back of the tube, closed up by six 
iron bars, abed ef, which cross each other. The middle bar 
is 4 inches broad, and i-|- thick at e, but is swelled so as to 
measure 5 broad, and l-l thick at 7 , where it is turned at rect- 
angles, and passes under the bottom of the tube. In this bar 
is a square hole, through which a pivot, or pin, passes from 
the inside of the tube, where it is confined by a square head, 
into the hole of the axle, A B, under which at the bottom it is 
keyed fast at C ; with proper washers between the joints to 
allow of a very smooth motion. 
The bar, eg, is of the same strength with el, and passes 
over it at e. It is bent at rectangles at c and g, so as to pass 
along the sides of the tube. The two bars, d m, fk, fastened 
upon eg, and afterwards turned down to the back, are 2 inches 
broad, and f- thick ; and are also bent at rectangles at m and 
k, so as to go under the tube : the remaining two- bars, b h, 
a i, cross the other three bars, with proper offsets ; and are 
bent at rectangles on both sides, that they may turn round the 
end of the tube, to go along the sides of it. At the crossing 
