HOT WATER. 307 
The pits.-must have a water-tight paved bottom, with a de- 
clivity of one inch in ten feet... The sides and covers of the 
channels are loosely jointed, and are permeable -by the 
steam. ' Stop-cocks are attached.to the pipes, so that .the 
supply of’vapor can be adjusted. Another mode. of adapt- 
ing steam to the production: of bottom heat may be seen 
in Mr. Macmurtrie’s Pine-Pit, to be afterwards described. 
Hot Water.—More recently the circulation of hot water 
in iron pipes or vessels has been successfully employed in 
producing artificial warmth. The temperature derived 
from this source has all the properties of steam-heat, with 
the following additional advantages: it is-more steady, be- 
ing less affected by changes of temperature in the open air 
than in houses heated by fire-flues, or even by steam-pipes; 
it is not liable to interruption by the bursting of vessels, 
and it is more lasting, as water does not cool so epee as 
aqueous vapor. 
The following explanation of the principle of the i. 
water apparatus is given by the late Mr. Tredgold, in an 
excellent paper in the Lond. Hort. Trans., vol. vii. “We 
may select the simple case of two vessels placed. on a: hori- 
Fig, 28. 
zontal plane, with two pipes to connect them; the vessels 
being open at top, and the one pipe connecting the lower 
