232 HEATING AS APPLIED IN HORTICULTURE. 



of resisting great pressure, unless very- 

 thick or supported with iron hoops. 

 This latter is, however, not a safe pre- 

 caution, on account of the unequal expan- 

 sion of the different metals. We have in 

 use, in the Dalkeith gardens, leaden boilers 

 which have stood the test of six years' 

 constant firing : they are of lead, weigh- 

 ing 16 lb. to the square foot, and have the 

 merit of heating rapidly, and are of 

 course not liable to oxidation, which all 

 iron boilers are. 



Malleable iron, being somewhat thinner 

 than cast-iron, heats sooner. Boilers 

 made of it are, however, much more 

 expensive, and, in consequence of their 

 natural tendency to oxidize, soon wear 

 out. Water impregnated with lime is 

 very destructive to malleable-iron boilers ; 

 and there are other matters often con- 

 tained in water equally pernicious to 

 them. The very best steam-boiler plates 

 we could procure have been used by us ; 

 and in no case has any malleable iron 

 boiler in general use lasted seven years ; 

 while we have cast-iron ones, exposed to 

 the same tests, which have lasted four- 

 teen years. Our preference has long been 

 in favour of the latter. 



For ordinary purposes, malleable-iron 

 boilers, where the pipes are laid level, or 

 little elevated above the boiler, may be 

 of J of an inch in thickness ; but such 

 boilers as are placed much below the 

 level of the pipes should not be less than 

 \ of an inch, as the pressure upon them 

 is very considerable. This is the usual 

 thickness for steam-boilers of the greatest 

 power. 



Earthenware boilers are longer in heat- 

 ing than metallic ones, but they have the 

 property of retaining the heat much 

 longer. As regards strength, they are 

 fit for the horizontal mode of circulation, 

 as the pressure in that case is very 

 trifling ; — we would not recommend them 

 when great pressure is apprehended. As 

 to their durability, so far as experience 

 has shown, they will last probably as long 

 as any of the metallic ones ; but this 

 entirely depends on the clay of which 

 they are formed. 



As regards the size of boilers, the 

 extremes of too large and too small ones 

 should be alike avoided. The advocates 

 for large boilers say they will keep their 

 heat much longer after the fire is extin- 



guished ; but it should also be kept in 

 mind that they take an equally greater 

 length of time to become heated; nor 

 will the circulation be so rapid as in one 

 of smaller dimensions. The opposite 

 extreme is sometimes fallen into by 

 having them too small; hence repul- 

 sion takes place between the iron and 

 the water, the latter not receiving a 

 sufficiency of heat ; and the higher the 

 temperature of the iron becomes, the 

 greater is the repulsion; — so that, when it 

 becomes red hot, water is completely 

 repelled, and the iron will scarcely com- 

 municate heat to it unless under great 

 pressure. 



The surface which a boiler ought to 

 expose to the fire should bear a proper 

 proportion to the length of pipe that is 

 to be heated by it. The following table 

 of proportions of surface which a hot- 

 water boiler ought to bear to the length 

 of pipe is given by Hood, assuming the 

 difference in temperature between the 

 pipes and the atmosphere to be heated to 

 be 140°— that is, the pipe being 200°, and 

 the air to be heated 60° : — 



J^tet^ 4 "^** 3-inch pipe. 2-inch pipe. 

 H wmXar* j 200 feet ' 266 feet 400 feet 



51 „ 



„ 300 „ 



400 „ 



600 



7 ~ 



~ 400 „ 



533 ~ 



800 



81 „ 



~ 500 „ 



666 „ 



1000 



12 „ 



„ 700 „ 



933 ~ 



1400 



17 ~ 



~ 1000 „ 



1333 „ 



2000 



There are causes, however, which alter 

 these proportions, — such as the construc- 

 tion of the furnace, the height of the 

 chimney, fuel, draught of flue, smaller 

 degree of temperature to be communi- 

 cated to the atmosphere than that as- 

 sumed above, &c. If the difference of 

 temperature between the hot water and 

 the air to be warmed be 120° or 100°, in- 

 stead of 140°, then the boiler in the 

 former case will heat one-sixth, and in 

 the latter one-third more pipe than is 

 stated in the table. That table affords, 

 however, the best data we have at present 

 for forming a correct estimate of the 

 proportion that should exist between the 

 boilers and pipes. 



On this subject Mr Ainger, in " Gar- 

 deners' Chronicle," makes the following 

 remarks : — " The proper size of the boiler 

 and fire-grate have been made the sub- 



