GLASS STEUCTURES AND APPLIANCES. 



327 



remarks apply to rooms, they to a certain 

 extent apply also to forcing-houses, at least so far 

 as velocity of circulation is concerned. Hence it 

 follows that the front ventilators of our hot-houses 

 should be equal if not superior in area to that of the 

 openings at the apex. The nearer to the ground- 

 line and the more divided the openings for the 

 admission of cold air, the fewer the currents; 

 but, unless a sufficient quantity of cold 

 air be admitted in this manner, a counter 

 cui-rent will descend and upset the cir- 

 culation, if it does not produce an in- 

 jurious effect upon the foliage. This 

 fact does not, however, 

 prevent our making 

 provision for abundant 

 ventilation along the 

 highest part of every 

 hot-house, as there are 

 periods which neces- 

 sitate and justify the 

 thi'owing of every light 

 and ventilator wide 

 open, and allowing 

 them to remain so during 

 the season of rest, or when 

 fruits, like the Peach, are 

 laying on colour and 

 ripening. Those who 

 have to ripen Peaches and 

 Grapes early in May, find it 

 useful to strip their houses 

 from base to summit as 

 soon as the crop is removed. 



Position of Ventila- 

 tors. — Front ventilators, 

 owing to the density of the 

 cold air, should be placed 



as near the ground -line of the house as possible. 

 Under some systems the fresh air is admitted by 

 underground drains, which can be carried a consider- 

 able distance before they enter the' house, but in all 

 cases they should enter in a horizontal line as shown 

 in Fig. 74, and so that the cold air will come into 

 contact with the hot- water pipes, which will warm 

 and rarify it before it reaches the foliage of the 

 plants. This mode of admitting air is not, however, 

 solely depended upon for reducing the temperature 

 in hot weather, as there are times when the gravity 

 of the external column is very nearly balanced by 

 the atmosphere of the house, and the rapidity of 

 motion is greatly reduced. But for keeping up a 

 constant movement by night and day, whenever the 

 front sashes are closed, this health-giving system is 

 w orthy of general adoption. If space admitted, in- 



Fig. 78.— Lantern, witli Side Light; 



stances could be given where this underground 

 ventilation not only kept plants in the rudest 

 health, but actually restored valuable collections 

 of Orchids to that state, after being half ruined 

 in the unwholesome atmosphere of double-glazed 

 houses. The front sashes hinged to the top plate 

 and worked by improved machinery open out- 

 wards, aU on the same level, and being of equal 

 ventilating capacity, an even stream 

 of air passes into the house from one 

 end to the other. The top ventilators 

 should also work in the same way and 

 on the same level. 



Apex Ventila- 

 tion. — The original 

 mode of ventilating 

 houses having fixed 

 roofs, by means of ven- 

 tilators let into the 

 back wall, immediately 

 below the roof, as in 

 Fij^. 75, and opening to 

 the external atmos- 

 phere at although a 

 very important step in ad- 

 vance, and still in exten- 

 sive use for night or wet- 

 weather ventilation, has 

 been gi-eatly improved 

 upon by the addition of 

 hinged lights, generally 

 from two to three feet in 

 depth, and running the 

 whole length of the roof. 

 Generally they are hinged 

 to a fillet fixed along the 

 tops of the rafters, and open 

 upwards by means of or- 

 dinary ventilating gear, 

 worked by a lever fi'om the back path. Fig. 76 is a 

 fragment of the top of a lean-to Yinery, showing a sec- 

 tion of the wet-weather ventilator, c, built like a semi- 

 circular flue in the thickness of the wall ; the hinged 

 lights can be opened up to the horizontal line, be 

 yond which they should not go, or the water, in 

 case of a storm, would flow into the house. Many 

 old conservative builders, for a considerable time, set 

 their faces against fixed roofs with continuous venti- 

 lating lights attached to the apex, but the moderate 

 cost of a fixed roof soon made them popular, and this 

 mode of glazing and ventilating is now accepted as 

 one of our greatest modern improvements. Span- 

 roofed houses of ordinary dimensions, and running 

 from east to west, require a ventilating light on the 

 south side of the ridge only ; but wheu very large, 

 and running in an opposite direction, i.e., with one 



