| STEAM. 303 
effectually steamed, that is, filled with the steam or vapor, 
and the warm moisture thus applied to the plants is ob- 
served to contribute remarkably to their health and vigor. 
To counterbalance these advantages, we are not aware of 
any defects, except such as may arise from the greater com- 
plexity of the’ apparatus, or at least its liability to disre- 
pair and accident. 
Steam is generated in a cast or mrpices iron boiler, of 
an oblong form, furnished’ with safety-valves, and heated 
by a smoke-consuming furnace. As in the common steam- 
engine, the boiler is supplied from a cistern above, and is 
made to regulate itself by a simple contrivance. In the 
feed-head is a valve, which is opened by the sinking of a 
float, which descends in proportion as the water is dissi- 
pated in steam; and, being balanced by a weight, whenever 
a sufficient quantity of water is admitted, rises again, and 
shuts the valve. As steam may ‘be conveyed, without ma- 
terially impairing its calorific powers, to the distance of 
several thousand feet, one boiler is sufficient for heating all 
the glazed houses which are ever erected together; but a 
second is generally kept in readiness, to act as an auxiliary 
in case of accident, or in very severe weather. Steam is 
conducted from the boiler in a single main pipe, or in two 
parallel pipes, which, according to Mr. Tredgold, may be 
only one inch in bore. The divarications of the pipes into 
particular houses are arranged somewhat in the manner of 
flues, and, indeed, are sometimes placed within these, or 
on them, when they already exist. ‘Thesé interior pipes 
are from three to six inches in diameter, in order to afford 
a greater radiating surface, and are supplied with sets of 
valves, to admit, regulate, or exclude the heated vapor, 
according to circumstances. 
The most perfect and extensive samples of steam ap- 
