362 Dr. Herschel's Description of a 
gular sides erected on the former, and held together by a nar- 
row platform on the top. Fig. 1 6. represents the bottom frame. 
Its length, ag, is 8 feet 10 inches, and breadth, gh, 2 feet 8 
inches. It is made of yellow fir deal, 4 inches broad, and 2 
inches thick. Six sets of brass rollers, in iron frames, con- 
structed as represented in fig. 17. by means of the two small 
screw-bolts, i k, are screwed under the frame, fig. 16. at c de 
f gh ; so that when this frame comes to be placed upon the 
front of the ladders at /3 y, or £ e, fig. 3. the 6 rollers in the di- 
rection m, fig. 17. will sustain the frame, while the other six, 
in the direction /, by embracing the flat sides of the ladders, 
which as has been described are turned outwards for this pur- 
pose, will prevent the frame from slipping off sideways, when 
it rolls up and down the ladders. 
Two such triangles as delineated in fig. 18. the wood of 
which is 3^ by 2-§- inches, are fastened to the frame, fig. 16 ; 
one, with the side np upon eg , the other upon dh. These 
being joined at the top by the platform of boards, screwed 
down to the supporters q r s, of that which is represented here, 
and of the corresponding one, which rests on dh, complete the 
bracket. 
Upon the platform are fixed palisades commencing at t, 19 
inches from q ; which turn the corner at the front s , and are 
continued so as to meet the middle platform of the gallery. 
The palisades over trs are strengthened and rendered steady, 
by a seat which is fastened against them, and supported from 
the floor by slight iron bars. 
The other double bracket, with its platform, palisades, and 
seat, which runs upon the right side of the front ladders, is in 
