234 HEATING AS APPLIED IN HORTICULTURE. 



the house, and the other into the external 

 air. The supply of cold air entering at 

 one end will take up and propel the heat 

 given out by the pipes, and carry it into 

 the house, with a portion of pure air 

 highly beneficial to the plants." This 

 would be all very well where the distance 

 between the boiler and the house to be 

 heated is not great; but if that distance 

 exceed 30 or 40 feet, it will be more 

 advisable to carry the pipes in drains as 

 completely air-tight as possible. 



Larger pipes can only be advantage- 

 ously employed when it is desirable to 

 keep up a certain temperature without 

 keeping up at the same time a constant 

 fire. This is seldom necessary, nor does 

 it appear that there is any advantage in 

 it as regards the economy of fuel. 4-inch 

 pipes are, therefore, the most proper size 

 to use for hothouses in general, although 

 3-inch ones may be employed in heating 

 greenhouses and pits. 



The form of pipes in general use is 

 very properly round, not only on account 

 of their more convenient manufacture, 

 but also because in that form they are 

 better adapted for giving out heat by 

 radiation. Flat pipes have been recom- 

 mended as presenting a greater upper 

 surface ; but this is rather a defect than a 

 merit, as the greater part of the heat 

 given out by them ascends perpendicu- 

 larly upwards, and will, therefore, pro- 

 duce less beneficial effects to the sur- 

 rounding atmosphere. Moreover, the 

 warmer such pipes are made, the loss of 

 heat is the greater; for it is well known 

 that the relative proportion which radia- 

 tion bears to conduction increases with 

 the temperature. 



One of the most important points to 

 be understood and acted upon, in the 

 arrangement of a hot-water apparatus, 

 is the proper setting of the boiler. Every 

 bricklayer professes to have a plan of his 

 own, when, indeed, very few of them 

 understand the first principles of the 

 matter. Boilers are, in general, set as if 

 the object of the setter had been to try 

 how much fuel could be consumed so as 

 to produce the least possible effect. It 

 is reasonable to suppose, that the larger 

 the surface of boiler exposed to the direct 

 action of the fire, the greater effect will 

 the fire have on it, the sooner will the 

 water become heated in it, and the less 



amount of fuel will be required to pro- 

 duce that effect. The earliest constructed 

 boilers were ill adapted for abstracting 

 the amount of heat contained in the fuel 

 burning under them ; and the directions 

 laid down by the earlier advocates for 

 hot-water heating were far from being 

 founded upon correct principles ; for, in 

 most of them, fully nine-tenths of the avail- 

 able surface was not exposed to the action 

 of the fire at all. The force of the fire 

 was made to act upon only a small 

 portion of their surface, and the boiler 

 in general was set much too close to it — 

 thereby preventing the radiant heat from 

 acting beneficially upon them. 



On the subject of setting boilers, Mr 

 Glendinning, in " Journal of the London 

 Horticultural Society," remarks : " Badly 

 designed boilers, when well set, often 

 answer beyond expectation, and even to 

 the astonishment of the contriver; where- 

 as, boilers constructed upon the best 

 principles, when improperly set, have 

 failed. In the latter case the heat is 

 not advantageously applied." The intel- 

 ligent authority just quoted says, in 

 continuation : " Mr Hood's method of 

 setting the arch-boiler cannot fail to 

 answer when properly carried out, as I 

 have amply proved. It appears to me, 

 however, that, for large houses, a greater 

 increase of the piers of the arch would 

 be an improvement, as thereby affording 

 more room for fuel. Beyond this, as 

 far as my own experience goes, it seems 

 to be the best source of warming yet 

 devised, combining, as it unquestionably 

 does, both efficiency and economy." 



The following illustrations, figs. 316, 



Fig. 316. 



317, and 318, from the work quoted 

 above, appended to Mr Glendinning's 



