HEATING BY HOT WATER. 165 
elevated 3ft. or more above the level of the pipes. From 
the bottom of this tank another pipe leads downwards 
again into the heating pipes, which must all be laid 
with a gentle downward inclination to the point where 
the return re-enters the boiler. If carefully carried out 
this method answers fairly well, though as there is 
naturally some loss of heat in the circulating cistern, 
etc., the results are scarcely as economical and effective 
as under the usual system. A simple arrangement of 
this kind is illustrated in Fig. 109. 
n=} 
Lt B > 
Fv 
n=n 
ed _= : 
ev 
Fig. 109. 
Every hot-water apparatus must be provided with a 
feed-cistern, as well as with one or more air-pipes, or 
cocks. The former is used for filling the pipes, etc., 
and it also allows for the expansion and contraction 
of the water. The air-pipes, which permit of any confined 
air or steam passing out of the heating pipes, and thus 
enabling them to become quite filled with water, must 
be fixed at all the highest points in the pipes where 
there is no other outlet. They are preferable to cocks 
