HEATING BY HOT WATER. 159 
The flow-pipe, or pipes, should always be fixed along 
the outside of the house, near the glass, this being the 
point where the heat is chiefly required, in order to exclude 
frost, etc. The old-fashioned plan of placing all, or nearly 
all, the piping in the pathways or centre of the house is 
now quite obsolete, as it has been conclusively demonstrated 
that it is at once much easier, more economical, and better 
in every way to keep the cold owt by heating the layer 
of air next the glass, than to warm the house from the 
interior, outwards. A current of warmed air constantly 
passing upwards from a hot pipe, just within 
the glass, effectually excludes all frost or cold, 
etc., and then the interior of the structure 
will be warm also, though in all houses of 
considerable width it is advisable to place 
two or more rows of piping—usually returns 
—along the body of the structure, at some 
distance from the sides. Even in stoves 
and forcing or propagating-houses, where yyig. ios, 
there is plenty of bottom—as well as top— 
heat in the body of the house, it is a capital plan to run a 
2in. pipe—a flow if possible—along just inside the eaves, 
near the wall-plate, as shown in Fig. 105. This will 
effectually prevent the ingress of anything like cold, even in 
the severest weather, and ensure a genial temperature at all 
times, but a valve or valves should be provided to enable 
the current to be cut off in mild weather, when less heat 
is required. 
In heating narrow span-roofed houses—of say 12ft. or 
less in width—a very common method is to run a 3in. 
or 4in. flow along each side, near the eaves, anda return 
