172 FIXED STRUCTURES FOR GROWING 



has also been circulated on the siphon principle with great success by 

 Mr. Kewley ; the advantage of which mode is, the rapid communica- 

 tion of heat along the whole length of the pipe, in consequence 01 

 which it is never necessary to raise the water in the boiler to so high a 

 temperature as by any of the other modes ; and hence this mode of heat- 

 ing is the most economical of all in the consumption of fuel. Fig. 143 



Fig. 143. " g> a correct 



& idea ot the system : 



^D) ace represent the 



two lea;s of the si- 



phon ; the upper 



Siphon mode of circulating hot water. ' , & 



at c, being that 



through which the heated water ascends, and the lower leg being that 

 by which it returns. The disadvantage of this system is, that after the 

 pipes have been some time in use they become leaky, and the slightest 

 leak, by admitting the air, instantly empties the siphon ; nor is the leak 

 easily discovered afterwards. The siphon mode of heating, were it not 

 for this disadvantage, would deserve the preference over most others. 

 Hot water has also been circulated in hermetically-sealed pipes by 

 Perkins ; but this mode is attended with great danger from the high 

 temperature of the water. All these, and other modes of heating, will 

 be found impartially examined in Hood's Treatise. 



A reservoir of heat is very readily formed in heating by hot water, 

 whatever may be the kind of apparatus adopted, by placing a cistern 

 or series of cisterns at different parts of the house, either close to or at 

 any convenient distance from the water-pipes, and connected with 

 them by smaller pipes, having stop-cocks to interrupt the con- 

 nexion at pleasure. When it is desired to heat the house with as 

 little loss of time as possible, all connexion between the pipes and 

 the reservoirs should be cut off by turning the stop-cocks ; and 

 as the house becomes sufficiently heated, the connexion ought to 

 be restored by opening the upper and under stop-cock of one cistern 

 at a time. In some cases, the cistern might be a long trough about 

 the bulk of a common flue, placed parallel with and close to the 



with stop- ffot-water pipe, and reserve cistern of hot water. 



cocks. Fig. 



145 is a cross section of the pipes and reserve cistern, which requires 

 no explanation. Where the circulating pipes are below the level of 

 the floor of the house, and where there is to be a raised pit for con- 

 taining plants, a tank or cistern might be formed under it of the length 

 and width of the pit, and of a depth equal to the distance between the 

 upper and lower heating-pipes ; and with this tank the pipes might 



