ON FURNA CES. 391 



gas from the furnace is burned, with a suitable admixture of air, in a 

 brick shaft in its interior ; and the products of combustion are drawn 

 down, throu h the mass of bricks, by the draught of a high chimney. 

 When this operation has been continued for three or four hours, the 

 greater part of the mass of brickwork, filling the body of the stove, is 

 at a uniform red heat ; and the heat decreases, thence, towards the 

 bottom, so that the gases passing off to the chimney are at a tempera- 

 ture not exceeding 150 C. ; no more than is sufficient to maintain a 

 draught. When the stove has thus been heated, the air, gas, and 

 chimney valves are closed ; and by opening the cold and hot blast 

 valves, blast is sent through it, in the reverse direction, passing first 

 among the nearly cool bricks in the lower part, next to the chimney, 

 and then over hotter and hotter surfaces, until before it reaches the 

 top of the stove, it has attained nearly to the temperature of the bricks 

 themselves. The hot blast thence rises through the remainder of the 

 column of chequer work, and passes down the brick shaft or combus- 

 tion chamber, and through the hot blast valve, to the furnace. By the 

 time that the current of blast, flowing through the stove, has begun to 

 cool down, sensibly, the uppermost courses of chequer work and the 

 walls of the combustion chamber, a second stove, which has meantime 

 been heating up, is put " on blast," and the first stove is heated again 

 in turn. By working a set of either two or three stoves, in this way, a 

 constant supply of hot blast is maintained ; the temperature of the 

 blast, between the beginning and the end of a shift, not varying more 

 than 50 or 100 C. When three stoves are worked together, two are 

 generally kept at a time "on gas," or heating up, and one "on 

 blast." 



The difficulty that was experienced in keeping the earlier forms of 

 the Cowper stove free from deposited dust, carried over from the fur- 

 nace, led to the introduction of a modification of it, by Mr. Whitwell, 

 that has also been extensively adopted. In this, the brickwork pro- 

 vided to take up the heat of the burning gas, and give it out again 

 to the blast, is arranged in the form of a series of parallel vertical 

 walls. These present less heating surface than bricks arranged on 

 the plan adopted in the Cowper stove ; and the stoves, being of less 

 height, take up also more floor space. The advantage claimed 

 them is that they are more easily cleaned. 



