WAEMING AND VENTILATINQ. 



259 



placed a few inches below the surface of the water where it is hottest, the lower, 

 or return -pipe, descending nearly to the bottom of the boilei', a small tube or air- 

 hole being placed in the upper pipe near to the bend at its highest elevation ; 

 the surface-water being the hottest, the heated particles are forced into the 

 upper pipe, after dispelling the air, get cooled, and descend again into the 

 lower pipe by the force of gravitation. 



702. Numberlessmodifications of these principles have been proposed at various 

 times, the chief modifications being in the form of the boiler. After many ex- 

 periments, Mr. Eogers was led to adopt a conical form of boiler, in place of a 

 cylindrical one, the interior or furnace resembling in shape the sugar-loaf cone, 

 supplied with coal from below. The boiler is also slightly conical. Cottam & 

 Halen, and Burbidge & Healey, have each of them produced boilers extensively 

 in use among gardeners ; the object in both instances being to present the 

 largest surface of the boiler to the flames ; and both, though by difierent 

 arrangements, carry the flameup the centre of the boiler. Mr. Thompson, of the 

 Whitton Nursery, Hounslow, has invented a boiler which presents an immense 

 surface to the action of the fire, and, fi-om the mode of setting, admits the 

 flame to play all round it, having projecting shoulders to extend the surface so 

 exposed. 



703. Where great heat is required, however, from moderate space, the 

 tubular boiler seems to have considerable advantages. Messrs. Weeks, Fowler, 

 and Kewley seem to 

 have been the earl- 

 iest adapters of this 

 system, now exten- 

 sively used in horti- 

 cultural buildings. 

 Weeks seems to 

 have adopted, and 

 even patented, the 

 well - known prin- 

 ciple of the dis- 

 placement of rare- 

 fied water by the 

 pressiire of the 

 denser cold water, 

 already explained ; 

 but his heating- 

 principle was a sys- 

 tem of pipes placed 

 round the fire, and 

 communicatingwith bievatxcn op messbngeb's boilees. 



the cistern, and with the warming-pipes. This he did at first in connection with 

 a boiler ; but the tubes were soon iuund to be perfectly efificient when ranged 

 round the fire without any boiler. This apparatus is now constructed of upright 



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