PLANTS, WITH GLASS ROOFS. 



171 



and those who wish to make themselves masters of 'the subject will have 



recourse to Hood's ' Practical Treatise on Warming Buildings by Hot 



Water.' The simplest form of applying this mode of heating is by 



having one boiler to each house in a recess in the back wall, or in 



some other situation where it will be out of the way, and an upper or 



flow-pipe proceeding from it on a level, with an under or return-pipe, 



also on a level. Fig. 140 will give an idea of this mode of circulation, 



a representing the Fi H0 



boiler, b a cistern at j^, 



the extreme end of i|S|pii8wi8H^^ 



the house to serve as j jji|^_ h \ 



a reservoir, and c the 



flue and return-pipes. A kot -^ ter apparatus for circulation on a level. 



When the water is to be circulated in pipes or on different levels and 

 above the level of the boiler, or on different levels but never below the 

 level 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 arrange- 

 ments (see Hood's Treatise, figs. 10 and 11, pp. 44, 45) may be em- 



Fig. 141. 



Fig. H2. 



Boihr and furnace for heating by hot 

 water in rising and falling pipes. 



Apparatus for circulating water below 

 and above the level of the boiler. 



ployed, or the form of open boiler shown in fig. 142 may be resorted to. 

 All depression below the level of the boiler should be avoided if pos- 

 sible. They involve a great loss of force, and frequently stop the cir- 

 culation. In this figure, a represents 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 J, 

 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 



