212 



HEATING AS APPLIED IN HORTICULTURE. 



tutes for tanks ; but they should always 

 be elevated from the ground, as here 

 shown, and fitted with portable covers, 

 so that the amount of evaporation from 

 them may be regulated. The flow passes 

 along one side of the house or pit, and 

 returns by the other, as shown at a and b, 

 fig. 282. 



A curious discovery has been made in 

 the garden of the London Horticultural 

 Society with regard to the correction of 

 dampness during winter in pits heated 

 by gutters ; it is noticed by Dr Lindley 

 in " Gardeners' Chronicle." " If," 

 he, " a b. 



Fig. 284. 



JZL 



in the 

 fig. 284, '"be the 

 gutters, and c d 

 the surface of 

 materials placed 

 above them, the 

 air will have no 

 considerable mo- 

 tion, water will lodge on the foliage, and 

 death will result with all soft and tender 

 plants ;" — this, of course, arising from 

 condensation; — " but the moment the line 

 d is made to 

 as in fig. 

 "the dim- 



slope," 

 285, 



culty is overcome. 

 By laying bare, or 

 nearly so, the gut- 

 ter a, which is the 

 flow gutter, and 

 raising the covering materials gradually 

 to d, a motion of the colder air takes place 

 from d to c, while at c the hotter air rises 

 up to the sashes, follows them, and, when 

 cooled, falls again to d; and this kind of 

 circulation going on incessantly, all damp- 

 ing off is effectually prevented." 



It does not appear to have been much 

 noticed how far heating by tanks, if 

 much steam be allowed to escape from 

 them, may tend to lower the temperature 

 of a house or pit instead of raising it; 

 yet such may be the case in consequence 

 of an excess of spontaneous evaporation. 

 " The process of boiling," Tomlinson 

 observes, " is by no means indispensable 

 to the formation and escape of steam or 

 vapour; for at all temperatures below the 

 boiling point, vapour is formed at the 

 surface of liquids, and escapes therefrom 

 by a process called spontaneous evapora- 

 tion. During the spontaneous evapora- 

 tion of wet surfaces, a considerable degree 



of cold is produced by the quantity of 

 heat rendered latent by the formation of 

 the vapour, and the heat is mostly derived 

 from the liquid itself, or the surface con- 

 taining it. By proper contrivances, water 

 may be frozen in consequence of the ab- 

 straction of heat during the rapid forma- 

 tion of vapour. When a person takes 

 cold from wearing wet clothes, the vapour 

 from the wet clothes obtains its heat from 

 his body, and the chilling sensation is 

 often the greater the warmer the air." 



We have already noticed that heat is 

 given off from bodies by the two pro- 

 cesses of radiation and conduction. These 

 are very different processes in the pro- 

 pagation of heat. By conduction the 

 heat travels through or among the par- 

 ticles of solid matter, and is gradually 

 communicated by one particle to another, 

 until the temperature of the body in con- 

 tact with the source of heat is elevated 

 more or less above that of the air. " When 

 heat is communicated to a fluid body, the 

 process is different. In consequence of 

 the great mobility of its particles, those 

 which first come under the action of the 

 source of heat, being raised in tempera- 

 ture, escape from its influence, and ascend 

 through the fluid mass, distributing a 

 portion of their acquired heat among 

 other particles on their way ; other par- 

 ticles immediately take their place, and 

 being heated, ascend in like manner, and 

 distribute their heat. By this process of 

 convection, as it is called, the whole of 

 the particles in a confined mass of fluid 

 come under the action of the heating 

 body ; those first heated escape as far as 

 possible from the source of heat, and be- 

 coming cooled, descend again to be heated, 

 again to ascend and descend. In this 

 way a circulation is maintained in the 

 whole mass of fluid. It is only by this 

 process of convection that air may be 

 said to be a conducting body ; for if a mass 

 of air be confined in such a way as to 

 prevent the free motion of its particles, it 

 ceases almost entirely to conduct heat, 

 and may be usefully employed to retain 

 it; as, in the case of double windows, 

 the enclosed mass of air prevents the heat 

 from escaping from the apartment, and 

 shields the glass which is in contact with 

 the warm air of the room from the cool- 

 ing action of the external air. Glass is a 

 very bad conductor of heat, and the cool- 



