206 FIXED STRUCTURES FOR GROWING 



both boilers being of the same dimensions, made of the same material, set in 

 the same manner, and with a fire beneath them of the same power. Yet 

 such is the case ; and this exactly on the same principle that we recommend 

 confining the sides of hot- water pipes, and supplying the air to be heated from 

 a distinct channel. Suppose we have a common boiler, such as is used in 

 common wash-houses, then place another boiler within it, of such a size as to 

 leave only a few inches between the inner boiler and the outer boiler all 

 round, and support it in this position by stays, as shown in fig. 146 ; let this 

 inner boiler have a hole in its bottom about one- 

 third of its diameter, and let its rim be two inches 

 below the level of the water to be heated. These 

 arrangements being made, and the heat applied 

 below, a circulation instantly takes place and con- 

 tinues, the water coming into contact with the 

 heated bottom and sides of the outer boiler, rising 



Fig. 146. Perkins s double boiler. 



rapidly to the surface, and descending through the 



inner boiler, which thus necessarily contains the coldest portion of the 

 liquid. (Gardener's Magazine, vol. xvi. page 325.) The heat commu- 

 nicated by the fire to the bottom and sides of the outer boiler, is rapidly 

 carried off by the current that is created, exactly on the same principle 

 that wind, which is a current of air, cools any body exposed to it more 

 rapidly than air at the same temperature but quite still. The under-ground 

 drains should either have vacuities at the sides and over the top to prevent 

 them from absorbing much heat, or they may be carried through the 

 bottom of the tan- pit, where there is one. In general, we would not cover 

 the heating-pipes, nor would we adopt the upright tubes which Mr. Penn 

 originally used, but has since dispensed with. There may be situations and 

 circumstances where it would be more desirable to have the heat of the pipes 

 or flues carried off by radiation with the usual degree of slowness rather than 

 by conduction ; such, for example, as when the attendant on the hothouse was 

 likely to be a long time absent, or when some danger from overheating was 

 anticipated ; and this can always be attained by covering the orifices by which 

 the air enters to the cross drains. It is proper to state, that at the present 

 time the opinions of a number of persons are against the use of air as a car- 

 rier of heat in hothouses, on account, they say, of the difficulty of maintaining 

 it in exactly the proper state of moisture. This, however, can be effected with- 

 out difficulty, by keeping the bottoms of the cross-drains covered with water, 

 or by having cisterns of water over the pipes, or both. A few years' experi- 

 ence is probably required to set the matter at rest ; in the mean time, the 

 reader who wishes to examine both sides of the question, may consult the 

 Gard.Mag. for 1840-41, and the Gard. C/tron., more especially an article by 

 Mr. Ainger, April 3d, 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 blos- 

 soms, 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 temperature, we make the following 

 quotation from Lardner's Cyclopedia : " The air which surrounds us is 

 generally at a lower temperature than that of the body. If the air be calm 



