Forty-feet Reflecting Telescope. ' 381 
We have seen that in fixing the ladders they were set at 
8 feet 2 inches distance in front, in order to permit the tele- 
scope to have a side motion, without displacing the whole ap- 
paratus, which is designed for a meridional situation. 
Every celestial object, when it passes the meridian, is then 
in its most favourable situation for being viewed, on account 
of the greater purity of the atmosphere in high altitudes. The 
advantage also of being able to direct the instrument, by 
means of the quadrant, to the spot in which we are to view 
the object, is considerable, in so large an instrument as the 
40-feet telescope. With unknown objects, it is likewise of the 
greatest consequence to be enabled, by a meridional situation, 
to ascertain their place. But, as a single passage through the 
field of view, especially with my examinations of the heavens 
in zones, would not have been sufficient to satisfy the curio- 
sity of an observer, when a new object presented itself, it be- 
came necessary to contrive a method to lengthen this interval. 
The tube, therefore, as we have seen, is made to rest with the 
point of support in a pivot, which permits it to be turned side- 
ways. 
Its diameter being 4 feet 10 inches, and that part which is 
generally opposite the ladders that confine it in front being 
about 35 feet from the pivot, it appears that a motion of 3 feet 
4 inches may be had, which to the radius of 35 feet gives up- 
wards of five degrees of a great circle. 
Several abatements must be made on account of the dispo- 
sition of the apparatus that gives this side motion, and the 
shortness of the ropes hi high altitudes ; but there remains, 
notwithstanding, a sufficient quantity of this lateral motion to 
MDCCXCV, 3 D 
