MECHANICS AND USEFUL ARTS. 71 



combustion below, but only between the bars of the grate, where the fuel is 

 so completely exposed to air, and near the mouth or top of the coal-box. The 

 unsatisfactory results of some other attempts to make such a fire, have been 

 owing hi part to the combustion extending downward hi the coal-box, be- 

 cause of air having been admitted below ; and the consequent melting and 

 coking of the mass of coal, so as to make it swell and stick, impeded the rising 

 of the piston. A remarkable quality of this fire is its little tendency to be ex- 

 tinguished. Even after nearly all the coal hi the grate has been consumed, 

 the air will descend into the coal-box and keep the fire there gently alight, 

 for a whole day without any stirring, and yet ready to burn up actively the 

 moment the piston is raised. For the purpose of allowing a graduated admis- 

 sion of air, hi order to ensure the maintenance of combustion with rather more 

 activity, there is a slide hi a small door at the front bottom of the coal-box. 

 Before lighting the fire, whatever ash remains with this form of combustion, is 

 removed off the piston. The fire is extinguished by allowing it to exhaust 

 itself, or by lifting out the few lumps of coke or caked coal which remain. 

 The charge should be such that enough cinder or coke may be left for the 

 smokeless lighting of the next day. By the means above described, the pro- 

 duction of smoke is obviated. We now come to consider the subject of the 

 waste of fuel in ordinary fires. Count Rumford showed that |ths of all the 

 heat produced in a common open fire passed up" the chimney with the smoke, 

 and therefore to waste ; and he appealed in corroboration to the experience 

 of those who use close stoves, which do not thus waste heat up the chimney, 

 and where much less fuel than is needed hi open fire suffices. As an exem- 

 plification of the above, Dr. Arnott gives the folio wing striking illustration: 

 " I have an enclosed fire which, for 14 years past has maintained for 24 hours, 

 from October to May, a continued temperature of 60 or more, accompanied 

 with good ventilation, by an expenditure of only 12 Ibs. of coal, or about one 

 fourth of that used in an open fire burning from 15 or 16 hours. The aper- 

 ture by which the fresh air enters the stove to maintain sufficient combustion 

 to warm the room is about three quarters of an inch in diameter." 



If tin's be compared to the aperture of a common chimney-pot, which has a 

 diameter often niches, and an area or size 150 times greater than the stove 

 in question, and we take into consideration the rapidity with which a column 

 of dense smoke fining that pot escapes from it when the fire is burning briskly; 

 and reflects further that such column consists entirely of the warmest air from 

 the room, blackened by a little pitchy vapor from the fire, there is proof of 

 prodigious waste, and room for reasonable hope that a saving is possible. To 

 see how a saving may be effected, the exact nature of the waste hi such cases 

 has now to be explained. A single mouthful of tobacco-smoke, on issuing, im- 

 me'diately diffuses itself so as to form a cloud larger than the smoker's head, 

 and soon would contaminate the whole air of a room, as would also the smoke 

 and smell of wood, paper, or other combustible burned in a room. Now, the 

 true smoke of a common fire is not the whole of what is seen issuing from the 

 chimney top, but only little dribblets or fits which shoot up or issue from the 

 cracks in the upper surface of coal which forms the fire. These fits, how- 

 ever, quickly diffuse themselves like the tobacco-smoke in the air around 



