40 



CIRCULATION OF THE ATMOSPHERE, CONSIDERED 



known. There are several modifications of this system, but the best 

 appears to be that by which a constant stream of fresh air is admitted 

 to a chamber outside the hothouse, where it is heated by passing over 

 the iron plate of a stove, and afterwards charged with moisture by 

 passing over a tank of hot water. It then enters the house, through 

 which its heat makes it rise and diffuse itself; but as it becomes cold 

 it sinks, and is finally conveyed by a cold-air drain to feed the fire of 

 the stove, the smoke being carried off by a chimney or flue. A 

 similar, though not so perfect a circulation is established in most hot- 

 houses without a drain at all. The roof and the floor are both con- 

 verted into drains for the circulation from or return to the pipes or 

 other sources of heat of the hot or cold air. Improved modes of 

 admitting fresh air below the heating apparatus in front or at the 

 back of houses are also general. 



The great point connected with the admission of cold air to hothouses 

 in cold weather is to charge it with moisture before it comes into 



direct contact with the plants. 

 The sketch, fig. 2, is the 

 section of a house heated by 

 pipes in the ordinary manner, 

 under the front shelves. The 

 arrows (numbered) indicate 

 the course of the current of air. 

 At No. 1 the air comes heated 

 from the pipes p, and ex- 

 tremely thirsty; at No. 2 it 

 finds moisture among the 

 plants, and rising from the 



damp and warm shelf (slate, of 

 Section of a hothouse, heated by hot water in CQurs ^ ftt NQ> 3 it hag 

 the ordinary manner. . , 



parted with some 01 its heat ; 



it is now supersaturated, and is parting with the moisture deposited 

 on the glass ; at No. 4 it is in the same state ; at No. 5 it has ceased 

 to lose heat or moisture ; at Nos. 6 and 7 the same ; at No. 8 it again 

 comes within the influence of the pipes, and is heated, becoming again 

 very dry. Now the air which descends to the floor (8) in the first 

 place, is a small and feeble current, and secondly, is nearly saturated, 

 so that it can take up little moisture ; and what little it does get is 

 because the floor, being slightly warmed by the radiation of the pipes, 

 warms, and at the same time moistens, the air ; but, nevertheless, the 

 air at No. 1 is anything rather than saturated. By placing evapora- 

 ting pans on the top of the pipes, to be filled or emptied at pleasure, the 

 air would start from above it, nearly saturated with water, and provide 

 the plants with a moist instead of an arid atmosphere. If fresh air 

 were admitted on the ground lines under the pipes at 8, it would 

 be warmed and watered at the same time, and intermixed with ascend- 

 ing currents of heated air. While, however, plant structures 

 kept at high temperatures require such expedients for charging the 

 air with moisturej they would prove simply mischievous in greenhouses. 



