154 GREENHOUSE CONSTRUCTION AND HEATING. 
is then allowed to fall into another set of pipes placed on 
a lower level, and is thus led back to the boiler, which 
it re-enters at the bottom, Here it again becomes heated, 
again rises, flows through the pipes, and returns, so that 
a constant current, or circulation, as it is termed, of the 
water in the pipes is maintained during the whole time 
that a fire is burning in the furnace. Fig. 99 shows a 
very simple arrangement, in which a is the boiler, B the 
‘‘flow’”’ pipe (which may however be separated into two, 
three, or more currents), o the farthest point (where an 
air-pipe D must be placed to allow any confined air or steam 
to escape), and E the “return” pipe, or pipes, through 
which the cooled 
and consequently 
comparatively heavy 
water falls and is 
conducted back to 
Fig. 99. the boiler. In the 
process of being 
heated from 32 degrees Fahr. (freezing point), to 212 
degrees (the boiling point) water increases in volume. 
considerably. 
A brief consideration of the above figure, and the 
system it illustrates, will plainly point out the necessity 
for two important conditions in the arrangement of 
all such heating apparatus, viz.: (1) that there must. 
be a rise, more or less, from the boiler into the 
first set of pipes, and (2) that the latter should 
be fixed with a gentle upward inclination in the 
“flow” from the boiler to the farthest point, and a 
corresponding fall in the ‘‘return” from the latter back 
