A REFRACTING TELESCOPE WITH A FLUID CONCAVE LENS. 45 
upper part of the wall ; and between these two are mortised 1 7 upright stan- 
cheons, each 2 inches by 4 ; and round the middle are framed other pieces of 
the same dimensions, viz. 2 inches by 4, cut also circular. These form the 
framing, which is lastly covered with inch boards properly tongued to keep 
out the wind and rain. Four windows are introduced, the size of the upper 
divisions, as seen in the plate, with a door not seen, being in the part supposed 
to be removed to show the instrument. The height of the boards is 6 feet 8 
inches above the brickwork, and 7 feet 3 inches above the floor. 
In another curb, exactly like those already described, are placed 12 iron 
rollers, which run on a circular plate of iron laid on the upper fixed curb. On 
the interior face of this moveable curb are also fixed 12 other iron rollers, which 
keep the curb to its centre by running against a plate of hoop iron, properly 
fixed to a fillet nailed to, but projecting above, the upper fixed curb. 
This moveable curb is, as we have seen, 16 feet 8 inches exterior diameter, 
and forms the base of the roof. The latter is formed of about 60 six-inch boards 
cut nearly diagonally, the broader end being securely nailed to the bottom 
curb of the roof, and the smaller to an upper curb 2 feet 8 inches in diameter, 
the boards being each 12 feet in length and securely dowelled together. Two 
rafters, each 12 feet long, 2 inches broad, and 4 inches deep, placed parallel to 
each other from the upper to the lower curb, and 1 6| inches asunder, form an 
opening for observation ; these are closed at other times by two shutters 
which turn on hinges in opposite ways the whole length of the roof. The joints 
of the boards of the roof are covered externally by canvass fillets of 1^ inch 
in breadth, secured by white lead in oil ; and lastly, the whole is protected by 
three thick coats of paint. The force necessary to overcome the friction of the 
roof is sixty pounds, and the motion is produced by an inch-and-a-half tarred 
rope passing externally round the moveable curb under the projecting eave- 
boards, which protect it from the weather : it then passes over two guide pul- 
leys, and descends to a drum about 8 inches in diameter, turned to the proper 
curve for surging ; it takes three turns round this drum, which is fixed to an 
axle that passes inside ; this axle carries a toothed wheel, which is worked by a 
pinion and handle, seen in the plate. The power thus gained is 8 to 1 ; so that 
to move the roof ought to require but a force of eight pounds : but in conse- 
quence of the extra friction, it practically requires twelve pounds : it may, how 
