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 wrought 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. These 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- 



