PLANTS, WITH GLASS ROOFS. 203 



of the bottom of the boiler, then a closed boiler is requisite ; or one open, 

 but carried to a height equal to that of the highest point in the line 

 of the pipes, as in fig. 141 ; and when water is to be circulated below the 

 level of the boiler, a closed boiler with particular arrangements (see 

 Hood's Treatise, figs. 10 and 11, pp. 44, 45) may be employed, or the form 

 of open boiler shown in fig. 142 may be resorted to. In this figure, a repre- 

 sents the boiler, b an open cistern at its top, in which the orifice of the 

 heating-pipe terminates. Now it is obvious that when the water passes 

 from the orifice of the boiler into the orifice of the pipe, the circulation 

 must go on from the difference in the specific gravity between the water in 

 the pipe at c, and that at d, provided that a small open pipe be placed at e, 

 to admit of the escape of the air which will accumulate in that part of the 

 pipe. Hot water has also been circulated on the syphon principle with 

 great success by Mr. Kewley ; the advantage of which mode is, the rapid 

 communication of heat along the whole length of the pipe, in consequence 

 of 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 heating 

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

 a correct idea of the 

 system: ac e repre- 

 sent the two legs of 

 the syphon ; the up- 



per leg Commencing Fi - 143 - $yP h de "f 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 syphon ; nor is the 

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

 for this disadvantage, would deserve the preference over every other. Hot 

 water has also been circulated in hermetically-sealed pipes by Perkins ; but 

 this mode is attended with great danger, and the heat produced is much too 

 high for the plants. All these, and other modes of heating, will be found 

 impartially examined in Hood's Treatise. 



499. 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 connexion 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 



pipe, as in fig. 

 144, in which 

 a is the pipe, 



b the cistern, 



^ ; * and c the con- 



Fig. 144. Hot-water pipe, and reserve cistern of hot u-ntar. nectin " pipCS 



with stop-cocks. Fig. 145 is a cross section of the pipes and reserve cistern, 



