PORTABLE, TEMPORARY, AND MOVEABLE STRUCTURES, 175 



trained against a wall with such a projection ; but the iron fastenings for 

 such a coping would require to be much stronger than for nine-inch copings, 

 on account of the greater power which the wind would have over them. 



464. Canvass Shades to Hothouses. — A very complete mode of rolling up 

 and letting down canvass over the roofs of hothouses was put in practice in 

 the kitchen-garden at Syon by Mr. Forrest ; and as it is equally well adapted 

 for covering awnings for tulip-beds or other florist's flowers, and for a va- 

 riety of other garden purposes, we shall here give such details as wUl enable 

 any intelligent blacksmith or carpenter to construct the apparatus. The 

 canvass is fixed to a roller of wood, fifty or sixty feet in length, the length 

 depending on the diameter of the pole or rod, fig. 115, a, and the toughness 



Fig. 115. Apparatus for rolling up and letting down canvass shades- 



of the timber employed, as well as the dimensions and strength of all the 

 other parts. On one end of this rod, and not on both, as is usual, a ratchet- 

 wheel, &, is fixed, with a plate against it, c, so as to form a pulley-groove, 

 between, to which a cord is fastened ; and about three inches further on the 

 rod is fixed a third iron wheel, about six inches in diameter and half an 

 inch thick, e. This last wheel runs in an iron groove, /, which extends 

 along the end rafter or end wall of the roof to be covered. The canvass or 

 netting being sewed together of a sufiicient size to cover the roof, one side of 

 it is nailed to a shp of wood placed against the back wall — that is, along the 

 upper ends of the sashes ; the other side is nailed to the rod, a. When the 

 canvass is rolled up, it is held in its place under a coping, ^r, by a ratchet, 

 h ; and when it is to be let down, the cord, of the roll is loosened with 

 one hand, and the ratchet cord, pulled with the other, when the canvass 

 unrolls with its own weight. The process of pulling it up again need not be 

 described. The most valuable part of the plan is, that the roll of canvass, 

 throughout its whole length, winds up and lets down without a single wrinkle, 

 notwithstanding the pulley-wheel is only on one end. This is owing to the 

 weight of the rod, and its equal diameter throughout. 



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