258 



HEATING AS APPLIED IN HORTICULTURE. 



furnaced boilers in use are defective as 

 smoke-consumers, inasmuch as the two 

 furnace tubes extend the whole length of 

 the boiler, so that the products of com- 

 bustion do not meet until their tempera- 

 ture has been reduced below the point of 

 ignition. Extended experience proves 

 that, as economic evaporators, Messrs 

 Galloway's boilers hold a high rank. They 

 are, however, better adapted for large 

 steam-boilers than for ordinary hot-water 

 ones; but, as we yet hope to see one large 

 hot-water boiler made to supply heat to a 

 large establishment of hothouses, such 

 boilers may yet find a place in gardens of 

 importance. 



Various other mechanical contrivances 

 have been tried, both for the purpose of 

 consuming smoke and also for feeding the 

 fire without manual attendance; but, from 

 their complication, they are found not to 

 work so satisfactorily as could be wished. 

 Still their respective inventors are en- 

 titled to praise for their ingenuity. The 

 examples we have given of smoke-con- 

 suming furnaces may be deemed suffi- 

 cient. 



The causes of opaque smoke being so 

 copiously produced by furnace fires are 

 thus detailed by W. Keld Whytehead, 

 C. E., in " Suggestions on Smoke-consum- 

 ing Furnaces," &c. — "The escape of car- 

 bon, the presence of which causes the 

 opacity of smoke, arises from imperfect 

 combustion of the fuel, which may be 

 occasioned by any of the following 

 causes. Over- rapidity of combustion. — This 

 is the most common cause of opaque 

 smoke, and defies the most careful stoker. 

 In other words, it is forcing a furnace — 

 i. e., compelling it to burn a larger quan- 

 tity of coals in a given time than it is 

 fitted to do. That is to say, a certain 

 proportion ought to exist between the 

 area of the furnace and the quantity of 

 fuel necessary to be consumed to evapo- 

 rate the required quantity of water ; or, 

 as engineers say, " a square foot of fire- 

 grate will only burn to advantage 10 lb. 

 of coal per hour." The reason of this 

 limitation is easy to be understood. Each 

 lb. of coal requires 2 lb. of oxygen for its 

 perfect combustion, to supply which 

 150 cubic feet of atmospheric air must be 

 passed through the fire. Now, all this 

 air has to pass through the spaces be- 

 tween the fire-bars, and sufficient space 



can only be obtained by having a propor- 

 tionate area of fire-grate. Thus, suppos- 

 ing the fire-bars are l|-inch wide at top, 

 and the spaces between them f ths of an 

 inch, it is obvious that out of the lj-inch 

 space in the width of the grate, only 

 § inch, or one-fifth of the whole, is avail- 

 able for an air-passage. But, even if 

 there be sufficient area of fire-grate, and 

 the fire-bars of proper proportions, there 

 may be want of draught in the chimney. 

 The passage of air through the fire is 

 entirely dependent upon the action of the 

 chimney, which is a species of pump; 

 and a chimney of insufficient diameter or 

 height will no more discharge a dispro- 

 portionate quantity of gas than a pump 

 would of water under similar circum- 

 stances. It is desirable that the chimney 

 should stand as near to the boiler as pos- 

 sible, because every foot of horizontal 

 flue causes additional obstruction to the 

 passage of the air, and diminishes the 

 useful effect of the chimney. It often 

 happens that the action of a good chim- 

 ney is neutralised by the malformation of 

 the flues surrounding the boiler. In this 

 case the remedy is easy, or the fault may 

 rest partially or entirely with the ignorant 

 fireman" We are afraid this defect will 

 be more difficult to remedy than " irregu- 

 lar firing and too much stoking, which are 

 the ordinary faults of the inexperienced 

 fireman. And the extreme of firing too 

 thick is as bad as too thin — the former 

 preventing the proper admission of air 

 through the bars, and the latter allowing 

 the air to pass through in excess, which 

 has the effect of cooling down the furnace 

 and preventing the perfect combustion of 

 the fuel. The effect of regular hand-firing 

 may be expressed in figures as follows : — 

 Suppose a furnace, the bars of which are so 

 proportioned as to admit sufficient air for 

 the proper combustion of 60 lb. of coal 

 per hour ; now, if once every minute we 

 could throw evenly over the fire 1 lb. of 

 coal, it is obvious that the quantity of 

 coal on the bars would be nearly invari- 

 able, because it would be supplied just as 

 fast as it was consumed, and the demand 

 for air would be constant. In hand-firing 

 a different operation takes place : the coal 

 is thrown on, say 15 lb. of coal every 

 fifteen minutes, and the consequence is, 

 that for one-half of the time too little 

 air passes through the bars, and for the 



