PLANTS, WITH GLASS ROOFS. 207 



and still, the particles which are in immediate contact with the skin acquire 

 the temperature of the skin itself, and having a sort of molecular attraction, 

 they adhere to the skin in the same manner as particles of air are found to 

 adhere to the surface of glass in philosophical experiments. Thus sticking^ 

 to the skin, they form a sort of warm covering for it, and speedily acquire 

 its temperature." Agitation of the air, however, ' continually expels the 

 particles thus in contact with the skin, and brings new particles into that 

 situation. Each particle of air, as it strikes the skin, takes heat from it by 

 contact, and being driven off, carries that heat with it, thus producing a 

 constant sensation of refreshing coolness." 



502. The boiler for heating by hot water need never be large, because no 

 advantage is gained by having a large quantity of water in it, further than 

 that of acting as a reservoir, which will be more conveniently and economi- 

 cally placed within the house. A boiler of small capacity, and with a large 

 superficies for the fire to act on, will be the most economical in first cost, and 

 also in fuel. " The extent of surface which a boiler ought to expose to the 

 fire should be proportional to the quantity of pipe that is required to be 

 heated by it ; " and Mr. Hood has calculated a table, which, like various 

 others in his excellent work, will be referred to by the intelligent inquirer, 

 or by the gardener who intends to direct the construction and putting up of 

 his own heating apparatus. By this table it appears : 



That 3^ square feet of surface of boiler exposed to the fire will heat 200 

 feet of 4-inch pipe ; or 266 feet of 3-inch pipe ; or 400 feet of 2-inch pipe. 



That 7 square feet of surface of boiler will heat 400 feet of 4-inch pipe ; 

 533 feet of 3-inch pipe ; and 800 feet of 2-inch pipe ; and so on in the same 

 ratio. 



" A small apparatus," Mr. Hood observes, " ought perhaps to have rather 

 more surface of boiler, in proportion to the length of pipe, than a larger one ; 

 as the fire is less intense, and bums to less advantage, in a small than in a 

 large furnace." (p. 71 .) 



503. The furnace, for a hot- water apparatus has also been subjected to 

 calculation by Mr. Hood. For generating steam, an extremely brisk fire 

 and rapid draught are required ; but a very moderate draught will suffice 

 for heating a boiler where the temperature of the water is rarely required 

 to be above 180 or at most 200. The following observations on the con- 

 struction and management of furnaces are valuable both with respect to a 

 hot water apparatus and the furnaces to common smoke flues. " The heat 

 should be confined within the furnace as much as possible, by contracting 

 the farther end of it, at the part called the throat, so as to allow only a 

 small space for the smoke and inflamed gases to pass out. The only entrance 

 for the air should be through the bars of the grate, and the heated gaseous 

 matter will then pass directly upward to the bottom of the boiler, which will 

 act as a reverberator, and cause a more perfect combustion of the fuel than 

 would otherwise take place. The lightness of the heated gaseous matter 

 causes it to ascend the flue, forcing its passage through the throat of the 

 furnace with a velocity proportional to the smallness of the passage, the 

 vertical height of the chimney, and the levity of the gases, arising from 

 their expansion by the heat of the furnace." (p. 7,7.) After giving a table 

 of the area of bars required for pipes of different dimensions and lengths, 

 Mr. Hood observes : " In order to make the fire burn for a longtime without 

 attention, the furnace should extend beyond the bars both in length and 



