354 
Dr. Herschel's Description of a 
that are bent, as in fig. 6. One of them, for instance, is bolted 
down, at a and b, fig. 3. upon the braces that hold the beam 
H in its place; upon which it is also screwed down, and makes 
a firm joint of the three pieces. A small entrance of a and b 
into H takes off the weight from the iron, and keeps the braces 
in their places, when bolted together. 
Two other braces, ii, kk, are added. Their use is evident 
in the horizontal motion of the telescope ; for that being ef- 
fected, as will be seen hereafter by means of the strong centre 
beam 1 1, the connection of the whole frame with this beam is 
completed by the pieces ii, AA, BB, CC, kk. 
Before I proceed to explain any other part of the work in 
this figure, it will be necessary to describe the construction of 
the ladders and their braces. 
Fig. 7. represents the front set of ladders, a (3 y $ e £ are six 
tapering halves of three large poles, or rather small masts, cut 
through the middle. Before the masts were cut they mea- 
sured between 11 and 12 inches at the bottom; but when 
they had been sawed through, the pieces were flattened on the 
front and back,' so as to be reduced to 8 inches at the bottom, 
and 5^ at the top, while the other dimension, or thickness, 
was left to its full extent. By trimming up and making them 
pretty equal, however, they became also reduced to about 
Sj and 5 inches in that direction. 
The length of the ladders is 49 feet 2 inches, and their con- 
struction is as follows. The top of each step is 9 inches from 
that of the one below it ; and, beginning 12 inches from the 
bottom, there are tw^o rounds and -one flat placed alternately, 
as far as 40 rounds, and 19 flats. In the place of the 20th 
flat is the centre of the meeting of the front and back sets of 
