HEATING BY HOT WATER. 155 
to the boiler. As a matter of fact, these two points are 
of the greatest importance, under all ordinary circumstances; 
with a deep boiler in proportion to its size the flow may 
run into the pipes on a level (see Fig. -100) and the 
circulation will still be maintained, and with a short length 
of piping only, these may also be laid on a dead level, and 
the water will circulate properly (especially if the ‘‘ flow” 
pipe is fixed above the ‘‘return,” instead of on the same 
level). But long lengths of piping must be given a 
moderate but regular upward inclination from the boiler. 
As little as 4in. of rise in 100ft., or lin. in 20ft. to 25ft., 
will suffice to enable 
the water to circu- 
late freely, but 5in. 
or Gin. per 100ft. is 
better, and should 
be allowed wherever 
possible. 
Again, the greater 
the rise from the 
boiler into the pipes 
the better. A boiler placed several feet below the level 
of the pipes will work much better, and the water will 
circulate more rapidly in the pipes than where the difference 
is only a few inches. In practice, however, a rise of from 
1ft. to 2ft. is sufficient, and answers well. 
In arranging and fixing the pipes there ought not to be 
any sudden dips in the flow, nor any rises in the return— 
the gradients must be easy and regular if the water is to 
circulate freely and rapidly ; these points are naturally of 
greater importance in large and complicated systems of 
Fig. 100. 
