HOT-WATER APPARATUS. . 35 
another lead pipe enters the union joint at the bottom of the boiler ; at B, the 
highest point of the whole system of heating- pipes, there is fixed on the upper side 
of the pipe a small air-pipe, whose other extremity is turned over into the supply 
cistern, so that any water casually thrown out there is returned into that cistern. 
The supply cistern, the bottom of which is not lower than the upper side of the 
pipe B, is placed over the reservoir, and communicates with it by a small pipe 
entering- the reservoir at the bottom. The steam valve is placed just above the 
supply cistern, having; a small basin or funnel round it, like those of the escape 
pipe of steam-vessel boilers, so that water thrown out, or condensed steam, may 
fall into the supply cistern. This escape of water or steam, however, can only take 
place when the water boils, which, if the boiler be properly proportioned to its work, 
can rarely take place. It is, however, a precaution which could not safely be 
omitted, though the danger of water being thrown out there is best obviated by 
loading the water with a small weight of two or three ounces, or a quarter of a 
pound on the square inch. 
The apparatus being filled through the supply cistern, the water enters the 
reservoir and thence fills both boiler and pipes, the air escaping from B by the air- 
pipe, until the water standing in the bottom of the supply cistern indicates that all 
is full. The fire being then lighted, the heated water flows into the iron pipes, and 
thence into the reservoir, till all having received one change of heat, it passes again 
through the boiler, acquiring fresh heat till all is nearly boiling. When the fire 
declines, a counter current takes place, the hot water from the reservoir rises to the 
pipes, where- as it cools it descends into the boiler, and thence into the bottom of the 
reservoir, till all be cool again — -having rendered up its heat into the pit or house. 
Fig. 5. 
Fig. 5 is a ground plan of the most convenient mode of applying this apparatus 
to a pit. The boiler and reservoir are inclosed in a little chamber of brick-work, 
