218 FIXED STRUCTURES FOR GROWING 



by which air is given, with wire netting, might be taken, which, while it 

 excludes wasps and flies in summer, would in winter act like Jeffrey's Respi- 

 rator, in abstracting the heat from the heated air which escaped, and im- 

 parting it to the cold air which entered ; or the double tube recommended by 

 Dr. Amott in his Treatise on Warming and Ventilating, might be adopted, and 

 probably with much more success than it can be in dwelling-houses. In hot- 

 beds it is customary to leave openings for the escape of moist vapour during 

 the whole of the night ; this is generally done by raising the sashes behind, 

 but, as by this mode the steam from the dung is sometimes driven in, some 

 gardeners have a narrow opening in the upper part of the sash, with a lid to fit 

 to it, hinged along the upper edge. The only difficulty that can occur in 

 ventilation, is in the case of houses heated to a high temperature by brick 

 flues not air-tight, and in which the atmosphere is unavoidably more or less 

 charged with noxious gases; but in this case, unless provision has been 

 made for heating the air before it enters the house, we know of no better 

 mode than opening the top sashes or ventilators more or less during the finest 

 part of every day ; and if the parts to be opened are covered with very fine 

 wire netting, it is presumed no chill will be given to the plants, and no greater 

 dryness created than can be readily moistened by the water evaporated from 

 the cisterns over the flues. The external air may be heated in the winter 

 season before it is allowed to enter the house, by enclosing a part of the pipes 

 or smoke flues, in a trunk or box with a communication at the lower part of 

 one end with the open air, and at the upper part of the other with the air of 

 the house. So long as the pipes are kept at a temperature considerably above 

 that of the house, fresh air will flow in, and a corresponding quantity will be 

 displaced by the accidental crevices of the roof. 



512, The agitation of the air in the house, with or without the introduc- 

 tion of fresh air, may be effected by cross drains in Mr. Penn's manner, 

 but omitting the covering of the pipes, and the upright tubes, and placing 

 the pipes in the front of the house, or round it when it is glass on all sides. 

 The cross drains, also, we would form of double sides and covers ; or of 

 earthenware tubes nine or ten inches in diameter, placed within other 

 earthenware tubes four inches wider, in order to retain a vacuity between 

 them ; or, for a similar reason, the tubes may be placed in brick drains. The 

 use of the vacuity is to prevent the loss of heat, w r hich would ensue from 

 its absorption by the sides of the drains, when they were at a lower tempe- 

 rature than the air of the house which passed through them. The inner 

 tube may be covered with water, as in the case of the common brick drain 

 used by Mr. Penn. 



513. Light is one of the elements of culture as essential as heat (278). 

 When the object is merely to grow plants without fruiting them, the propor- 

 tion of glass may be small, provided it be pretty equally distributed over the 

 roof; but when the object is to produce flowers and fruit, the proportion of 

 glass to the wood or metal of the roof ought to be greater. In nurserymen's 

 houses for growing plants, the most economical size of panes, or width be- 

 tween the sash bars, is five inches or six inches by three inches ; and the ordi- 

 nary breadth for houses in which plants are to be flowered is from seven to 

 nine inches. The panes in the latter case are generally made square, and in 

 glazing one is made to lap over the other from one-eighth to one-fourth of an 

 inch. In general one-eighth of an inch is quite sufficient ; the broader the 

 laps, the greater is the quantity of water which they retain, and the more 



