HEATING BY HOT WATER. 167 
connection close to the boiler. This acts as a kind of 
safety-valve, relieving the pressure caused by any sudden 
rush of water or steam from the boiler, and preventing 
the unpleasant and injurious ‘‘kicking” noise often 
experienced in such tases, as well as allowing any confined 
air or steam to escape instead of being driven into the 
pipes. 
The pipes are frequently laid on proper chairs, formed 
of cast-iron (see a in Fig. 110), and these are sometimes 
fitted with rollers, as at B in the illustration, to allow of the 
pipes moving easily as they expand, but in market-growers’ 
houses they are usually supported by means of brick 
piers of the requi- 
site height, placed 
here and there— 
generally one to B 
each (9ft.) length— nd sr C bh 
and these are found Fic. 110. 
to answer the pur- 
pose quite sufficiently well. The bricks must, however, 
be laid on a firm bottom, and bedded in cement rather 
than mortar. 
Amount of Piping Required for Different Tem- 
peratures.—Structures in which a comparatively high 
temperature has to be maintained, must, naturally, be 
fitted with more piping, in proportion to their size, than 
houses from which frost is to be excluded only, or where 
nothing more than a cool greenhouse temperature is 
required. 
A good many rules have been formulated, from time 
to time, and by various authorities and writers, for 
