HOT-WATER BOILERS AND PIPES. 181 



produce much inconvenience, but neutralize the effects of what 

 might otherwise be an efficient apparatus. 



The nearer the rules here laid down for regulating the size 

 of boilers are acted upon, the more efficient will be the working 

 of the apparatus. There is no advantage whatever gained by 

 using a larger boiler than is necessary to heat the pipes to their 

 maximum temperature, — even though this temperature may 

 never be required, — for, as the return-pipe should (if the appara- 

 tus be working right) bring in a fresh supply as rapidly as the 

 flow-pipe takes it away, the boiler is . always kept full. It 

 may be observed, that the circulation vdll be more rapid from a 

 minimum boiler than from a maximum one, — that is, from a 

 boiler whose capacity is rather below the proportion ; while a 

 boiler whose capacity is above the proportion of the pipes, has a 

 slower circulation ; and for all horticultural purposes, — though 

 the former has some little advantage in the time of heating — 

 the latter is decidedly to be preferred. 



In the following section, (Sect. V.,) further information will 

 be found on boilers, etc., where different methods of heating, in 

 practical operation, are figured and fully described. 



We may here state, in regard to the material for boilers for 

 horticultural purposes, that cast-iron boilers, if properly made, 

 will last much longer, and be also somewhat cheaper in the first 

 instance, than malleable-iron ones, be the plates ever so good ; 

 the principle of durability resting on the former not being 

 injured by oxydation so much as the latter. Iq both cases, 

 however, the durability depends very much on the kind of water 

 used ; that least liable to form a deposition on the boiler being 

 the best. 



2. Size and arrangement of hot-ivater jnpes. — Some contro- 

 versy has arisen, among engineers, gardeners, and others, respect- 

 ing the size of tube most suitable for the purposes of heating 

 hot-houses. 2, 3, 4, 5, and 6 inch pipes have been used, and 

 experiments instituted respecting the merits of each ; from which 

 it has been found that 4 inch pipes radiate more heat than any 

 of the other sizes ; and, consequently, the 4 inch pipes are now 

 most generally used 



