Forty-feet Reflecting Telescope . 351 
meter, and the inside one 21 feet ; these measures are rec- 
koned from the centre of one wall to the centre of the other. 
They are 2 feet 6 inches deep under ground ; two feet 3 
inches broad at the bottom, and 1 foot two inches at the top ; 
and are capped with paving stones about 3 inches thick, and 
12-f broad. Fig. 2. represents a section of one of them. 
These walls were brought to an horizontal plane by means 
of a beam turning upon a pivot fixed in the centre of the circle, 
which had a roller under it at the end. Upon this beam and 
over the roller was fixed a spirit level, to point out any defect 
in the walls ; and by correcting every inequality that could be 
perceived, they were by degrees brought to be so uniformly 
horizontal that the beam would roll about every where upon 
them without occasioning any alteration in the bubble of the 
spirit level. 
The timber of the groundwork (see fig. 3.), in the construc- 
tion of which it was necessary to join strength to lightness, is 
put together in the following manner : three principal beams, 
A A, B B, C C, are extended from south to north, when the 
telescope is in a meridional situation. They are 43 feet 2 
inches long, 6 inches broad, and 6 inches thick. The distances 
of the centre of the two outside ones is 17 feet. Within one 
foot of the ends of them are bolted down the cross beams 
D D, EE; which serve as a foundation to the two sets of lad- 
ders. These cross beams are 19 feet 2 inches long, 12 inches 
broad, and 6 inches thick ; and by way of additional strength 
two more, F F, G G, of the same breadth and thickness, are 
bolted against the sides of the former, resting also upon the 
longitudinal beams, but these are rounded off at the ends, as 
marked in the figure. 
Zz 2 
