176 FIXED STRUCTURES FOR GROWING 



covered with water, or by having cisterns of water over the pipes, or 

 both. A few years' experience is probably required to set the matter 

 at rest ; in the meantime, the reader who wishes to examine both sides 

 of the question, may consult the * Gard. Mag.' for 1840-41, and the 

 4 Gard Chron.,' more especially an article by Mr. Ainger, April 3rd, 

 page 212. Our opinion is, that the power of producing motion in the 

 air, even though it should be only wanted occasionally, and obtainable 

 at an extra expense of heat, is of so much value for setting blossoms, 

 equalizing heat and moisture in some cases, drying up damp in others, 

 or producing a feeling of coolness, that no plant-structure of large 

 dimensions, and where fire heat is employed, ought to be without it. 

 To explain the manner in which the motion of heated air in hothouses 

 produces a sensation of coolness, without being altered in its tempera- 

 ture, we make the following quotation from Lardner's * Cyclopaedia' : 

 " The air which surrounds us is generally at a lower temperature than 

 that of the body. If the air be calm and still, the particles which are 

 in immediate contact with the skin acquire the temperature of the 

 skin itself, and having a sort of molecular attraction, they adhere to 

 the skin in the same manner as particles of air are found to adhere to 

 the surface of glass in philosophical experiments. Thus sticking to 

 the skin, they form a sort of warm covering for it, and speedily acquire 

 its temperature." Agitation of the air, however, "continually expels 

 the particles thus in contact with the skin, and brings new par- 

 ticles into that situation. Each particle of air, as it strikes the 

 skin, takes heat from it by contact, and being driven off, carries that 

 heat with it, thus producing a constant sensation of refreshing cool- 

 ness." Less importance is attributed to confining the pipes and under- 

 ground drains than formerly. If the source of heat is placed mostly 

 in one part viz., the front of the house, the roof and cooler portion of 

 the house will induce a rapid circulation of air, apart from drains or 

 other expedients. 



A boiler of small capacity, and with a large superficies for the fire 

 to act on, will be the most economical in first cost and also in fuel. 

 " The extent of surface which a boiler ought to expose to the fire should 

 be proportional to the quantity of pipe that is required to be heated by 

 it ;" and Mr. Hood has calculated a table, which, like various others in 

 his excellent work, will be referred to by the intelligent inquirer, or 

 by the gardener who intends to direct the construction and putting up 

 of his own heating apparatus. By this table it appears 



That 3j square feet of surface of boiler exposed to the fire will 

 heat 200 feet of 4-inch pipe, or 266 feet of 3-inch pipe, or 400 feet 

 of 2-inch pipe. 



That 7 square feet of surface of boiler will heat 400 feet of 4 -inch 

 pipe, 533 feet of 3-inch pipe, and 800 feet of 2-inch pipe, and so on 

 in the same ratio. 



u A small apparatus," Mr. Hood observes, " ought perhaps to have 

 rather more surface of boiler, in proportion to the length of pipe, than a 

 larger one, as the fire is less intense, and burns to less advantage, in a 

 small than in a large furnace " (p. 71). 



