COVERING GLASS ROOFS. 



553 



oil has been said to destroy such fabrics, 

 every third or fourth year. They lasted 

 for many years. They were put on every 

 evening, from the beginning of November 

 till the beginning of March, and taken off 

 every morning, the whole operation taking 

 one man about five minutes, the house 

 being 50 feet long, with two sets of shut- 

 ters — one for the lower, range of roof 

 sashes, and another for the upper. A 

 pine-pit adjoining, 40 feet long and 12 

 feet broad, was also covered in the same 

 manner, only the shutters in this case 

 were as long as the width of the roof, and 

 in one piece. Fuel was expensive, which 

 led to this precaution, and the saving was 

 considerable ; besides, the plants were not 

 subjected to the unnatural excitement 

 they would have been exposed to, if 

 strong fires had been kept up during the 

 night. In fact, these shutters were made 

 upon far more correct principles than any 

 we have since seen. The framework fitted 

 close to the styles, and the top and bottom 

 rails of the sashes, so that little or no air 

 could find its way between them and the 

 glass ; and hence their great utility. The 

 volume of air thus enclosed, being in a 

 quiescent state, became a most powerful 

 non-conductor, which would not be the 

 case were the air allowed to circulate be- 

 tween them and the glass, as is the case 

 with the houses at Hamburg alluded to 

 above. 



The woodcut annexed, fig. 789, exhibits 

 a mode of covering and ventilating prp- 



Fig. 789. 



posed by Mr Moore, in the " Journal 

 of the Horticultural Society," vol. ii. 

 p. 28 : — " I am aware," he says, " that 

 night covering is a generally acknow- 

 ledged benefit, and that it is in some 

 cases acted on ; but it should be more 

 universally and more systematically fol- 

 lowed up. To assist in bringing the 

 principle of night covering into more 



VOL. I. 



universal application, is the purpose of 

 the following suggestion. I need not 

 stop to show how night coverings prove 

 beneficial. It is sufficient to state, that 

 whatever prevents the radiation of heat 

 from the interior to the exterior atmo- 

 sphere, through the conducting agency 

 of the glass, decreases in the same ratio 

 the amount required of applied heat, and 

 hence saves the plants from being ex- 

 posed to unnecessary excitement. The 

 principle upon which a covering acts 

 most effectually is that of enclosing a 

 complete body or stratum of air exterior 

 to the glass — this body of air being 

 entirely shut away from the surrounding 

 outer atmosphere; and as air is a bad 

 conductor of heat, the warmth of the 

 interior is by this means prevented from 

 passing to the exterior atmosphere ; or, in 

 other words, the exterior atmosphere being 

 prevented from coming in contact with 

 the glass, cannot absorb from the interior 

 any sensible proportion of its heat. To 

 secure this advantage, however, the cover- 

 ings must be kept from contact with the 

 glass ; and they should extend on every 

 side where the structure is formed of 

 materials which readily conduct heat, 

 such as glass and iron." An improve- 

 ment connected with this subject, and 

 which is probably seldom acted upon, is 

 to have the covering to fit so accurately 

 as to exclude the external air, " and then 

 to have a series of ventilators provided, 

 to stand open during the night. The 

 stagnation of the internal atmosphere 

 would thus be prevented, in consequence 

 of the interior air, and the air between the 

 glass and the covering, being of different 

 degrees of density, owing to their being 

 differently charged with heat. It will be 

 understood that, as here shown, the side 

 and end shutters (the latter not indicated) 

 fit into grooves, the upper groove being- 

 attached to iron pins, and thus fixed at a 

 proper distance from the building, with- 

 out obstructing the passage of air along 

 the enclosed space, and that on the lower 

 side being so fixed as to exclude the 

 external air in that direction. The top 

 or roof shutters also run into a groove 

 along the ridge of the roof, and at the 

 lower end are fixed close down to the 

 top of the side shutters, fastening with a 

 button. Each of the shutters should 

 have a projecting fillet fixed on one side, 



4 A 



