136 PORTABLE, TEMPORARY, AND MOVE ABLE STRUCTURES. 



one side of it is nailed to a slip of wood placed against the back wall 

 that is, along the upper end of the sashes ; the other side is nailed to 

 the rod, a When the canvas is rolled up, it is held in its place under 

 a coping, #, by a ratchet, h and when it is to be let down, the cord, t, 

 of the roll is loosened with one hand, and the ratchet cord, &, pulled 

 with the other, when the canvas unrolls with its own weight. The 

 process of pulling it up again need not be described. The most valu- 

 able part of the plan is, that the roll of canvas, throughout its whole 

 length, winds up and lets down without a single wrinkle, notwith- 

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

 weight of the rod, and its equal diameter throughout. 



A very simple mode of placing canvas on roofs is to have it 

 on rollers in the usual way. Then divide the roller space into three 

 equal parts, nail a strong cord on to the top of the roof at each of these 

 points, and bring the cord down under the roller to the front of the 

 house. Then carry the cord back over the canvas, run it through a 

 pulley at the point where it is fixed at top, return the two ends to the 

 bottom, and join them together. The roller will bring the blind down 

 the roof by its own momentum. All that is needed to roll it up is to 

 pull the cord in front and hook the slacks on to a pin. To bring it 

 down, unloose the cord, and give the blind a sudden jerk. This plan 

 gets rid of all rack wheels, &c., and cannot fail to keep the canvas square. 



The common hotbed frame is a bottomless box, commonly six feet 

 wide, and three, six, or eighteen feet in length, formed of boards from 

 one to two inches in thickness. The height at the back may be two 

 feet, and in front one foot. The bottom should be level, so that the 

 sides and the sashes laid on the frame may slope from back to front. 

 A three- light or three-sashed frame is divided by two cross bars or 

 rafters, so as to leave a space between them from two feet nine inches 

 to three feet for the width of the sash. It is placed either on the open 

 ground, or on a mass of heating material, according to the purpose for 

 which it is wanted, and, excepting for particular purposes, facing the 

 sun. As the great object of frames is to increase temperature without 

 excluding light, the soil on which they are placed, or the dung-bed or 

 other means of heating which they cover, ought to be as dry as possible, 

 either naturally or by artificial drainage ; and the glass ought to be 

 clear, and so glazed as to permit as little air as possible to escape be- 

 tween the laps. When common crown glass is used, small panes are 

 found to be less liable to breakage than large ones of this kind of glass ; 

 but when the sheet window-glass is used, from its greater thickness, 

 the panes may be two or three feet in length, without much danger of 

 breakage. The boards used for the frame should be of the best red 

 deal ; and if, after being prepared for fitting together, they are 

 thoroughly dried on a kiln, and afterwards soaked with train-oil in 

 the manner which we have before described for preparing wooden props, 

 the duration of the frame will be greatly increased. All frames and 

 sashes, when not in use, should be kept in an open airy shed, and there 

 raised from the ground a few inches by supports of bricks or other 

 suitable materials. In gardens where cucumbers and melons are grown 



