HEATING. 



217 



the two liquids are kept separate; consequently 

 the height of the two columns will be unequal, be- 

 cause the weights of the two liquids are unequal, 

 the pure spirits of wine being, bulk for bulk, 

 nearly one-fourth lighter. Now, although water 

 of an equal temperature would stand at the same 

 height, yet if cold water were put in the one leg 

 and hot in the other the heights would be un- 

 equal, because cold water is heavier than hot; 

 and although the greatest difference between 

 the coldest and hottest water is less than that 

 between water and spirits of wine, still it is 

 sufficient to produce an inequality of pressure. 

 If both legs were completely filled, and heat 

 applied, say to the leg a, the water would over- 

 flow; but if the two legs were connected at top 

 as well as at bottom, and one side were kept hot 

 and the other cold, the water would constantly 

 ascend in the warm leg and descend by the cold, 

 and thus a circulation would be established. 



The primary cause of the motion is the heat 

 of the fire rendering the water in the boiler 

 specifically lighter than that in the pipes; and 

 when this effect takes place the law of gravity 

 immediately comes into operation, so that the 

 actions of flow and return are simultaneous. 

 The greater the difference between the tempera- 

 ture of the water in the boiler and that in the 

 return-pipe, the more rapid the circulation, and 

 the converse. 



Furnaces. — The best furnaces are those in 

 which the fuel is most effectually consumed; it 

 is scarcely necessary to mention that they should 

 be adapted for a strong fire, in order to counter- 

 act intense frost; but they should also be con- 

 structed for slow burning when only a little 

 heat is required. 



The combustion of ordinary heating materials, 

 it is well known, requires a certain amount of 

 air. In furnaces this is usually supplied through 

 the bars, and occasionally in part through the 

 furnace-doors. Sufficient air should be admitted 

 to supply the necessary amount of oxygen, with- 

 out which the fuel would not burn; more than 

 sufficient is injurious, for it robs the fire of a 

 portion of its heat, and carries it up the chim- 

 ney. It is therefore necessary to have the 

 power of regulating the admission of air, and 

 consequently that the furnace-door, as well as 

 the ash-pit door, should fit perfectly. Hinged 

 doors are used for small boilers, and sliding 

 door-fronts for larger boilers. In fig. 280 the 

 doors move on rollers on an iron rod. The 

 frame of the opening projects a little outwards 

 towards the base, so that the weight of the door 

 partly rests upon it; and the more the door is 



moved the closer it fits. By these doors the air 

 can be regulated to a nicety ; and a good stoker 

 will soon find out how much opening he should 

 allow. If he require much heat he must increase 

 the quantity of fuel, and in proportion the supply 



Fig. 280.— Sylvester's Furnace-doors. 



of air. Once produced, the heat must distribute 

 itself in some way, and the object to be attained 

 is, to transfer as much of it as possible to the 

 water in the boiler. The latter ought to be set 

 so as not to obstruct combustion, and should 

 have a large surface directly exposed to the 

 radiation of the fire. 



Boilers. — Of these there are many kinds, some 

 of them very complicated; but of any two that 

 may prove equally effective, the simpler form is 

 to be preferred. We should be inclined to give 

 the preference to the horse-shoe or saddle form 

 of boiler, with a large surface for the fuel to act 

 upon in a direct manner, and such, too, is the 

 conclusion arrived at by most practical men 

 after a long experience of boilers of different 

 forms. 



The plain Saddle-boiler is well known, and if 

 properly set its working may always be safely 

 depended on, though its action may not be so 

 quick as that of some others. It may be safely 

 used to heat up to 1000 feet of 4-inch piping, 

 though from 750 feet upward the check end 

 and flued saddle may be used in preference, as 

 being more powerful in proportion to the size 

 of furnace and fuel burnt. 



There are numerous modifications of the 

 saddle-boiler, and most of them are efficient. 



One of the best is the Gold-medal Boiler, 

 represented by figs. 281 and 282, and so called 

 from its having gained the gold medal in the 

 boiler contest at Birmingham in 1872. It is of 

 wrought-iron, and has a water-way back: the 



