VENTILATION. 



275 



Acting upon the theory here laid down, 

 Mr Williams of Pitmaston ventilated his 

 melon frames by keeping the south side 

 of them open day and night— this opening 

 being merely covered with a fine 

 Fig. 358. * gcreen of fly-wire, painted black 

 H (fig. 358.*) The air passing 

 / \ through the wire-screen is di- 

 / B vided much in the way that 

 / « water is divided in passing 

 /.. through the rose of a watering- 



|| pot ; and, according to Mr Wil- 

 li hams' views, it receives a degree 

 of heat from coming in contact with the 

 black-coloured wire. One peculiarity in 

 this mode of ventilating is, that the wire- 

 screen is set to the exact angle of the roof. 

 " This screen receives the rays of the sun 

 from 10 a. m to 3 p.m. all summer long ; 

 it becomes heated to 80° or 100°, and 

 consequently heats the air that passes 

 between its interstices. By raising the 

 sashes at the back, a very powerful cur- 

 rent of air is established. The thermo- 

 meter ranges from 80° to 90° below the 

 leaves in a sunny day ; and, in short, the 

 atmosphere is as hot as is experienced in 

 the southern parts of Italy, with almost 

 as much ventilation as if growing in the 

 open air." — Gardeners' Chronicle, and Jour- 

 nal of Horticultural Society, 



The admission of cold air, either in 

 large or small quantities, into forcing 

 frames or houses is extremely dangerous, 

 and the dread of its bad effects often pre- 

 vents sufficient air being given. If air 

 could be brought to nearly the same tem- 

 perature as that within the house, it 

 might be applied in larger quantities, and 

 be of great service to the plants. This is 

 not easily done without depriving it of 

 too much of its natural humidity, an evil 

 to be guarded against. The primitive 

 mode of suspending a thin bass mat over 

 the opening of a frame in early spring is 

 good — it causes a regular dispersion of 

 the air without absorbing much of its 

 humidity ; so also is a plan often adopted 

 by ourselves — namely, covering the front 

 openings of hothouses with thin canvass, 

 and leaving the lights or ventilators open, 

 or partially so, in the coldest weather. Fine 

 fly-wire would be more lasting; and it 

 may be painted black, were it only for 

 appearance' sake. Whatever means may 

 yet be employed for producing ventila- 

 tion upon correct principles, it is evident 



that the present mode is extremely faulty. 

 We have elsewhere observed that ventila- 

 tion has hitherto been considered more 

 as a matter relating to the regulation of 

 the temperature than to the admission of 

 a regular and well-diffused supply of air, 

 so necessary to the existence of vegetable 

 as well as of animal life. 



"Ventilation in hothouses is required 

 to serve a double purpose — to renew the 

 atmosphere, or exchange a portion of the 

 external for the internal air, and thus to 

 give a gentle motion or current amongst 

 the plants. If this be done judiciously, 

 it may be practised at all times or sea- 

 sons with but little waste of fuel. Motion, 

 and consequently change, exist in every 

 natural climate, and cannot be entirely 

 withheld from plant-houses without a 

 corresponding loss of vigour and compact 

 growth. The other purpose of ventila- 

 tion is to prevent an injurious increase of 

 temperature during hot weather, or sud- 

 den bursts of sunshine. Although the 

 principles of ventilation are simple enough 

 to be understood by all gardeners, I may 

 mention there should always be one set 

 of ventilators, or sliding sashes, at the top 

 or upper part of the house, and another 

 near the floor, where the newly-admitted 

 air may, by passing over a part of the 

 heating surface, have its temperature gra- 

 dually raised to that of the house. By 

 giving more air by the lower than by the 

 upper ventilators, it will be gradually dis- 

 charged at the top of the house, with but 

 little cold draught amongst the plants. 

 If the upper ventilators only are opened, 

 we shall experience counter currents and 

 draughts of cold air. The lower should 

 be comparatively numerous, in order that 

 small quantities may be admitted by 

 each, thus preventing sudden gusts of 

 cold air at one place. It is apparent that 

 all our contrivances for warming and re- 

 gulating temperature and humidity in 

 confined atmospheres will have no effect 

 in deteriorating the air, and that its ori- 

 ginal constitution will remain unchanged. 

 As the most important functions of vege- 

 table life are the decomposition of car- 

 bonic acid gas, and the assimilation of 

 carbon under solar influences, the source 

 of the latter, and other elements which 

 plants derive from the air, will be present 

 in the usual proportions. Nevertheless, 

 the plants may not be able to assimilate 



