554 



DETAILS OF CONSTRUCTION. 



so as to shut close over the adjoining one. 

 The shutters themselves should of course 

 be made of light framework, strengthened, 

 where necessary, with small iron rods. 

 The material used for covering them may- 

 be the asphalt felt, or strong brown paper 

 coated with tar." 



Of all protecting materials for covering 

 pits and hothouses, mats are undoubtedly 

 the worst, and are expensive both to pro- 

 vide and apply. Patent asphalt, upon 

 Crogan's or M'Niell's principle, is pro- 

 bably the best, when fitted up as shutters 

 of the same size as the sashes — being both 

 a non-conductor, and almost waterproof 

 at the same time ; or it may be render- 

 ed perfectly so by coating it over with 

 asphalt and pitch in a hot state, and 

 dusting it over with fine sharp sand 

 before it becomes hard. Its weight, how- 

 ever, is liable to break the glass, unless 

 provision be made, as above, for prevent- 

 ing it coming in too close contact with 

 it ; nor is it adapted for rolling up in the 

 manner of canvass, being liable to break 

 on account of its stiffness. 



A . lighter and more flexible material 

 may be found in thin canvass, such as is 

 used for covering ricks, and purchased 

 for from 8^d. to Is. per yard. It may be 

 rendered quite waterproof by being cover- 

 ed with resin and melted fat in about 

 equal quantities, adding about one-fourth 

 of bees'-wax, the whole laid on when quite 

 hot. The bees'-wax, being antiseptic, pre- 

 serves the canvass, and gives consistency 

 to the other ingredients, and prevents 

 their becoming soft by heat, as w T ell as 

 keeps them pliant. Mineral pitch has 

 been tried, but not with successful re- 

 sults. 



The best materials for excluding cold 

 and retaining heat must be non-con- 

 ductors of both heat and cold. Hence 

 the wooden shutters used in Russia over 

 their double glass roofs, and the straw or 

 reed mats employed by the Dutch and 

 Germans, are well fitted for the purpose, 

 so far as non-conduction is concerned. 

 They are, however,, both rather unwieldy 

 to handle, but, from natural circum- 

 stances, absolutely necessary in those 

 countries. Woollen cloth, or felt, if 

 rendered waterproof, appears to us the 

 most suitable for our climate ; and the 

 mode of applying it must be .regu- 

 lated by circumstances, as our roofs are 



of so great a variety of form. Where 

 rafters are used, we would recommend 

 framed shutters, to fit into them over the 

 sashes; and where rafters are dispensed 

 with, coverings in large pieces, mounted 

 on rollers, would be the best, as re- 

 quiring little time to put on or take 

 off ; for much of the success of covering 

 will depend on this matter. Waterproof 

 canvass is the next best material to 

 w T oollen cloth. 



That much heat is saved by covering 

 glass roofs is very clear ; but what that 

 amount is, is not easy to calculate. Dr 

 Wells, in his experiments on dew, found 

 that a cambric handkerchief, supported 

 6 inches above the ground, caused the 

 ground at that spot to be 8° warmer in a 

 clear frosty night than the surrounding 

 ground that was not so protected. Fol- 

 lowing this fact, many cultivators place 

 the covering at some distance from the 

 glass, so as to admit a body of air to 

 intervene ; while others lay the covering 

 close upon it, calculating that, by this 

 means, both radiation and conduction 

 will be very much prevented,— as with 

 glass the cooling process arises from these 

 two causes, the former being increased 

 by a clear sky, and the latter being much 

 affected by the force of the wind. Thick 

 coverings have been much used in cold 

 countries — thickly formed straw mats, 

 and even boarding, being often had 

 recourse to, and, with such protection, 

 they estimate a saving of two-thirds of 

 the ordinary loss of heat. In accordance 

 with these views, 4 Mr Percival, in " Gar- 

 deners' Chronicle," says : "By a good 

 covering of wood or thick straw, I have 

 no doubt fully two-thirds of the ordinary 

 loss of heat may be prevented; and I 

 think it may be fairly assumed in prac- 

 tice, that the saving will be quite equal 

 to one half — that is to say, that the loss 

 of heat from glass so covered will only 

 be one half what it would otherwise be, 

 provided such covering fits tolerably close 

 to the frames, without allowing any cir- 

 culation of air between the glass and the 

 covering. Of course, also, if the top only 

 of a pit or house be so covered, the saving 

 must only be calculated for the portion 

 so treated, and not for the whole house." 



Impermeable or waterproof canvass 

 may be prepared in any of the following 

 manners : — 



