PLANTS, WITH GLASS ROOFS. 



211 



times as much as the pipes. Reservoirs may be made of iron, but, though 

 rather less expensive, they are so heavy and unwieldy that they could hardly 



Fig. 152. Rogers's hot-water reservoir. 



be used; and the expense of attaching the pipes would greatly exceed the 

 cost of copper. Mr. Rogers has a seventy-two gallon reservoir, a cylinder 

 four feet long by two feet in diameter, which cost complete, with two one- 

 fourth-inch union joints, £5. 5s. 



The foregoing directions will enable any intelligent gardener to plan and 

 put up an apparatus for himself. 



It remains to say something respecting fuel : any sort except wood and 

 caking coal may be employed. The best of all is anthracite or Welsh coal, 

 but a little coke is necessary to light it ; the next best is coke ; and next to 

 this, cinders. Mr. Rogers arranges them thus, in the order of their strength ; 

 but for ordinary purposes nothing is better than cinders : nay, even coke breeze, 

 or small refuse coke, the value of which is next to nothing, may be burnt in 

 these furnaces, but in that case they require eight or ten feet of chimney. 

 Where it is required to produce strong heat rapidly, coke must be employed ; 

 but it is not a good fuel to maintain heat, as it allows too much draught, and 

 burns away. Welsh coal has not this fault, and is a very durable fuel, 

 peculiarly well suited to these boilers. When the fire is first lighted it should 

 be allowed to burn brisk and clear, till the fuel in the bottom is well ignited; 

 it may then be filled up to the throat of the furnace, when it will last through 

 the night. In filling, care, of course, must be taken that the fuel is not so 

 small and dusty as to stop the draught. Where cinders are used they should 

 be well sifted. The proper management of these boilers may be best secured 

 by explaining the principle upon which they are constructed. As fuel can- 

 not be consumed without air, if a furnace be constructed of considerable 

 depth, and filled with fuel, and air be admitted only at the bottom, that fuel 

 alone is consumed which lies immediately on the bars, and first receives the 

 draught of air. The fuel above, provided it transmits the air, becomes red- 

 hot, or nearly so, but does not consume until that below it is destroyed. In 

 this manner, one of these conical furnaces being lighted and filled with fuel, 

 that portion in the upper part of the furnace which cannot burn absorbs the 

 heat of the burning fuel below, and radiates or transmits it to the water on 

 every side. So perfect is this absorption of heat, that for several hours after 

 the furnace has been filled up with cinders, though there may be a fierce fire 



