Forty -feet Reflecting Telescope. 367 
feet more than the length of the intended tube ; and the con- 
cavity of which was formed by the same radius as that of the 
tube. 
The sheet being let down, it rested upon the hollow gutter ; 
for so we may call the machine that was placed under it. Six 
moveable segments of a whole cylinder, or circular arches, 
about 3 feet wide each, which had been prepared, were now 
brought upon the sheet and placed at proper distances from 
each other. By these the sheet was pressed down upon the 
foundation, so that no injury could be done by walking upon 
it. The beams which held the pulleys were now brought 
close together ; which being done, we hung the pulleys of one 
upon the hooks of the other beam, so as by that means to cross 
the cords which held the sheet. In this operation we slack- 
ened only one of the cords at a time, the rest being sufficient 
to keep the whole up. 
The beams were now again separated, and the cramping 
hooks by the crossing of the cords drew the two sides of the 
sheet together. 
Here I must take notice, that the circular inside supports, 
which resembled the machines upon which arches of brick- 
work are built, were cut in two in the middle, as in fig. 27. ; 
some part of the circumference being taken out, that when 
they were laid down upon each other they might not fill the 
tube. Four long wedges, abed, *in opposite directions, were 
confined two and two in the notches efgb; and similar ones 
at the back. By driving them in very equally, the upper half 
of the arches might be forced up so as to swell to the full ex- 
tent of the tube. 
When all this was properly arranged, and the arches lowered, 
3B 2 
