VENTILATION.] HORTICULTURE 231 To this end the walls must be built hollow, and the pipes placed near the bottom of the cavity, and supported on cast-iron chairs placed on the top of the foundation course of material. This method was first exemplified by Air Atkinson in the gardens of the duke of Bedford at Woburn Abbey in 1828.
- g The Tank System of heating garden structures was introduced
is. some years since by Mr W. E. Rendle. It consists in circulating hot water in broad shallow tanks instead of closed pipes. It will be obvious that as the water is to flow along these open (or only loosely covered) conduits, they must themselves be level, and the boiler must be either at the same level or below it, the connexion between the two being made in either case by hot-water pipes of suitable length. The tanks are formed of wood, brick, stone, or cast-iron. When of wood, they require to be made of good sound plank, not less than 2 inches thick, and properly jointed, and they are usually covered with slates. Stone or brick tanks require to be lined with ?. thick coating of Roman cement, while stone, slate, or brick pavement may be employed for covers. The cast-iron tanks have covers of the same material. When one tank only is employed, as for example in supplying bottom heat to the front bed of a propagating pit, it should be constructed with a division along the centre, running to within about a foot of the end, which should be left open to allow of the water passing to the other side ; the flow pipe from the boiler should be connected with the end of the tank on one side of the central division, and the return pipe for leading the water back to the boiler should be connected at the same end on the other side of the division, a sufficient number of pipes being also connected with tho flow ami return pipes to heat the atmosphere to the temperature desired. It is convenient to have valves fixed in the flow and return pipes, so as to shut off bottom or top heat as may be required. In a larger house a tank may run along the front, across the end, and along the back to the end whence it started ; and this may be either divided along the centre as in the former case, the water flowing the whole distance on one side the division and returning on the other, or a broad tank may occupy the front and back, arid be connected at the end by an ordinary 4-inch pipe, the flow from the boiler being united with the front tank, and tho return carried from the back tank into the boiler. The tank system of heating is perhaps one of the readiest modes of supplying bottom heat to plunging beds either for propagating or for growing plants, though the same end is practically attained by running some of the ordinary 4-inch pipes connected with the heating apparatus through a tank which can be charged with water when necessary, and emptied when heat is not required. When the plan is used for supplying both top and bottom heat, provision must be made to prevent too much of the steam or vapour from passing into the house ; and, on account of the danger of having too much damp at certain seasons, it is perhaps preferable to have a separate set of pipes for the supply of atmospheric heat. r Solar Heat. The rays of the .sun reflected from walls and other surfaces become a source of artificial heat. This species of heat, however, is materially affected by the admission of the air necessary to the health of the plants. Solar heat, if properly regulated by ventilation, is of immense importance in the ripening of all the finer fruits, as well as for the abundant development of blossoms in the case of flowering plants. In the orchard house, fruits are ripened by the influence of solar lieat alone. To secure as much solar heat during the day as possible, the ventilators should be opened early in the morning, and closed early in the afternoon. tila- 16. Ventilating Apparatus. The object of ventilation is not so much to aid the respiration of the plants as to carry off noxious vapours and to regulate the moisture and temperature. The mechanical agitation both of the atmo sphere and of the foliage caused by a free circulation provided it be not too cold or too rudely admitted, has also a beneficial influence on the health of the plants. All ventilation, observes Mr M Intosh (Book of the Garden, i. 271), "is founded upon the simple principle that cold air is heavier and has a tendency to sink downwards, while hot air is light and rises to the top. At first sight it may appear that, for the purpose of ventilating any building, it is only necessary that holes should be made at the bottom of the apartment for the air to enter, and other holes be placed in the upper part for the air to escape. Practically, however, ventilation is far from being so simple an affair," since currents of air are very difficult to direct or control, to moderate, or even to regulate with very great nicety. Too frequently air is given only during the day, and is excluded at night, often with an increase of fire-heat. Judicious horticulturists will reverse this, since plants require fresh air by night as well as by day, and in all countries cool nights succeed even the hottest days. The mode of ventilating plant houses formerly in use was by letting down the top roof sashes ; this was often supplemented by having the front or upright sashes also made movable, and more or less widely opened either outwards or laterally during the daytime. It is much more economical, however, and equally efficacious, to have the roof sashes fixed, and the top ventilation effected by means of a lantern in the case of a span-roofed house, or by horizontal ventilators near the top of the back wall in a lean-to. The front ventilation may be effected by openings in the front wall opposite the heating pipes, by which means the cold air is warmed, which is the best plan in all houses where a high temperature has to be maintained, and is especially necessary in forcing-houses, on account of the risk of injury to the foliage from cold air ; or the front sashes may be made movable, which is better in temperate houses. In the case of pits, where there are no front sashes provided, the ventilators are sufficient for the purpose. The sliding down of one sash over the other, as once generally and even now frequently practised, greatly augments the shade in oblique sunshine, and is strongly objectionable on that account. The lantern mode of con struction obviates this, but in a lean-to, and even with span roofs, it is better to have shorter movable sashes, hung so as to open by being elevated at the lower edge instead of sliding one over the other. A very slight elevation is sufficient for the egress of vitiated air. A successful plan of warming the fresh air, recommended many years ago by Mr T. Moore (Journ. Hort. Soc., i. 110), consists in passing the air after its admission by front ventilators through a heated chamber separate from the tank used as the heating medium, but admitting of com munication with the tank chamber if necessary for the purpose of supplying moisture. The warmed freeh air is then led out in front of the tank, and carried forwards by the circulation up the slope of the roof, descending near the back wall to the Moor, whence it is sucked into the heated chamber, mingling with the fresh air as it enters to repeat the circuit. A ventilator in the back wall provides the means of egress when this is required. Another plan, well adapted for forcing-houses, consists in fixing from end to end, below the hot- water pipes used for heating the structure, a zinc pipe of 6-inch or 8-inch diameter, and perforated with small holes, one end of the tube passing through an outer wall, and being fitted with a valve which can be wholly or partly closed at pleasure. Other means of accomplishing the same end may be adopted to suit parti cular cases. In order to secure the circulation of the confined air during the night, and thus to prevent an injurious rise of the temperature, and also to economize fuel, it is of ad vantage where practicable to use shutters. These should consist of a light frame, readily movable, and fitted so as to slide readily in grooves on a skeleton roof ; and they should be covered with asphalted felt, or strong brown paper coated with tar, which is much used in Germany for covering purposes, and is both durable and cheap. This should form a close outer covering, the ventilators being- set open at bottom and top. Formerly all ventilation had to be effected by the hand unaided, each sash being opened or shut separately, a matter of some urgency on the sudden outburst of sunshine. In all good ranges of glass, and in detached houses also, the work is now effected by machinery, many ingenious combinations of which are in use for this purpose. 17. Hygrometry. For the healthy growth of plants, Hygro- the regulation of the moisture is as necessary as the regu- metry.
lation of the heat. A considerable degree of moisture is
