A REFRACTING TELESCOPE WITH A FLUID CONCAVE LENS. 45 



upper part of the wall; and between these two are mortised 17 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 16| 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 boai'ds 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- 



