The Economic Production of Artificial Heat. 403 



the atmosphere. Beech contains the. least, and fir the most 

 moisture. The peat of commerce frequently contains twenty- 

 five per cent, water, and when carefully dried, at least ten per 

 cent. Coal usually contains one or two per cent., but very 

 much more if exposed to the atmosphere and rain, particularly 

 when it is in dust or small pieces. By exposure to the air, char- 

 coal absorbs at least ten per cent, water, which causes flame 

 during its combustion, the water being decomposed, and car- 

 buretted hydrogen formed. In wet weather coke will absorb 

 seven per cent, of water. It is clear that, in purchasiug fuel by 

 weight, its hygrometric water may seriously affect the quantity 

 in reality obtained, and therefore its commercial value. 



Primage and leal-cage cause heat to be wasted. The former, 

 which consists in water being mechanically suspended in the 

 steam — not to speak of its other inconveniences — involves a 

 loss of the heat exoended in raising such water from the tern- 

 perature of the feed to that of the issuing steam. The less 

 pure the water in the boiler, on account of mud or greasy 

 matter, the more rapid the evaporation, the greater the loss 

 from this source : and hence the primage is greater with loco- 

 motive than with fixed boilers. Keeping the boiler clean, 

 and allowing the steam a sufficient space for deposition of the 

 water, reduce the waste, from this cause, almost to nothing. 

 Leakage of water or steam leads to a waste of all the heat car- 

 ried off by the w r ater or steam which escapes. 



The heat required to produce a draught in the chimney is 

 another source of loss, the amount of which depends on the 

 quantity of gaseous matter passing off, and the temperature at 

 which it is emitted. The minimum of quantity is the transmis- 

 sion of just so much air through the furnace as will burn the 

 fuel ; and the minimum of temperature is that which exceeds, 

 by only a few degrees, the temperature of the water, which is 

 being converted into steam of the required pressure. The tem- 

 perature at which the products of combustion cease to act on 

 the boiler, or other body to be heated, is a matter of consider- 

 able importance : and any loss arising from it is dependent on 

 the elevation of temperature of the waste products, and on their 

 weight and specific heat. It is not unusual for these to pass off 

 at a temperature of 60CP ; and this, with twice as much draught 

 as is necessary, would cause a loss of nearly twenty-seven per 

 cent, of the whole heat. A diminished draught, by raising the 

 temperature of the products of combustion, increases the heat 

 of the escape current also ; and therefore a diminution of 

 the draught to a nearer correspondence with what theoretically 

 may be required, does not economize the heat as mucL as might 

 at first be supposed. But the loss from this latter cause does not 

 counterbalance the advantage, since the higher the temperature 



