312 PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 



can practicably be made the means of admitting the external air 

 into the house at an increased temperature, and thereby creat- 

 ing a motion in the internal atmosphere, is a question which, as 

 yet, we are unable to prove from experience, although we mean 

 to take an early opportunity of testing the plan which we are 

 about to describe. 



2. The best material which we have seen used for this pur- 

 pose is canvas, or any other kind of strong coarse cloth, painted 

 with two or three coats of pitch, wax, and oil boiled together, and 

 applied in a warm state to the cloth ; this makes an efficient 

 and durable covering. Asphalte felt is also used extensively 

 in England and Germany for this purpose. This latter mate- 

 rial is fixed on light. wooden frames, about the size of a sash, or 

 larger, as may be found convenient ; and for covering frames 

 and pits it answers admirably, as it is quite impervious to wet, 

 and if taken care of, will last for some years. But for covering 

 the roofs of large houses, we would decidedly prefer the cloth, 

 which can be more easily drawn ofT and put on, and, if well 

 painted, will be as impervious to air and wet, as wooden shut- 

 ters, or asphalte frames, and will be cheaper than either. 



Suppose, then, that a glazed cloth, of the requisite dimensions, 

 is prepared. We would provide means for securing it against 

 wind, by loops, etc., and fix on parallel strips of wood over each 

 rafter, about nhie inches from the glass. The cloth should be 

 made to fit quite close at the top, and to reach the ground on 

 all sides of the house, which, formed of conducting materials, or 

 side-pieces, must be made to fit closely over the over-lapping 

 edge of the upper one, and the lower edge secured against the 

 admission of air. The house is now in a case, impervious both 

 to air and water, and enclosing a stratum of air, which gradu- 

 ally becomes warmer than the external atmosphere, and efiectu- 

 ally prevents the latter from abstracting the heat from the inte- 

 rior of the house. Then let there be square holes made along 

 the cloth, near the bottom, say one for each alternate light, each 

 aperture made about ten inches square, and provided with a 

 shutter of the same material, to close it when necessary. All 

 these apertures, or any number of them, may be opened, accord 



