242 



HEATING AS APPLIED IN HORTICULTURE. 



the free radiation of heat from most 

 bodies, varying, however, with their radi- 

 ating powers. On a good radiator its 

 effects are the most injurious; on a bad 

 one, less so; but its use should be avoided 

 as much as possible in all cases where the 

 free radiation of heat is the object in 

 view." Ainger, also an excellent authority 

 on such matters, treats this subject lightly, 

 however, and observes, "The colour and 

 texture of the pipe surfaces, with refer- 

 ence to radiating powers, have been some- 

 times insisted on; but these are refine- 

 ments of little value. The differences in 

 radiating power are very trifling till we 

 arrive at polished metallic surfaces, and 

 these are not likely to be adopted. Pipes 

 are usually covered with black paint ; but 

 almost any material, and any colour, will 

 do equally well." Our own practice is to 

 use anti-corrosive paint, which is pre- 

 pared without any white lead whatever, 

 and has decidedly the advantage over all 

 other paints of resisting the effects of 

 oxidation. The late Mr Lymburn ob- 

 serves, on the radiating power of pipes, 

 "When the radiating power of lamp-black 

 is 100, polished iron is estimated at 15; 

 and though rough cast-iron, and oxidised 

 on the surface, will be much more, still it 

 would add greatly to the effect to have 

 them coated with lamp-black." 



Sir Joseph Paxton appeared to be at 

 one time in favour of elliptical pipes cast as 

 thin as possible, and also observes, ("Maga- 

 zine of Botany," vol. ii. p. 253,) " that thin 

 sheet-copper ones are far superior, and in 

 the end the most economical." He also 

 says, " Zinc, though otherwise an excellent 

 material, is not well suited for elliptical 

 pipes ; it becomes so soft at the tempera- 

 ture of boiling water that they soon, the 

 lower ones especially, change their shape 

 and become nearly round." This, how- 

 ever, we think, is caused in a great de- 

 gree by the expansive force, or pressure 

 of the water in them. Zinc pipes we 

 cannot recommend, as they are so liable 

 to accidents and decay. 



Leaden pipes may be very advantage- 

 ously employed for heating pits, &c; but 

 they should not exceed 2^ inches in 

 diameter ; and where a considerable tem- 

 perature is required, two or more courses 

 of them may be used. Their advan- 

 tages are, a total absence of oxidation 

 and rapidity in heating; and, from being 



in long lengths, fewer joints will be re- 

 quired. 



Earthenware Pipes have been employ- 

 ed in many cases as a substitute for cast- 

 iron ones, and even earthenware boilers 

 have been used. An instance of the 

 former occurs in the gardens of J. 

 Greenall, Esq., employed to heat a pit 

 between 30 and 40 feet in length. A 

 portion of these pipes, which are of 

 large dimensions, is introduced into a 

 vault below, for affording moist bottom 

 heat. These have openings in their 

 upper surface for the escape of a mode- 

 rate quantity of steam; the others are 

 conveyed round the pit in the usual 

 manner, and supply dry heat. In con- 

 necting earthenware pipes with metallic 

 boilers, it is advisable to connect the 

 pipe with a block of stone, perforated at 

 one side to the size of the pipe, and at 

 the other, to the size of the nozzle-pipe 

 of the boiler. This last joint should be 

 put together upon the union-screw prin- 

 ciple, to prevent leakage from a difference 

 in the expansion of the materials. Where 

 earthenware boilers are used, this pre- 

 caution is unnecessary. 



Earthenware evaporating pans are very 

 frequently used. The annexed diagram, 

 fig. 321, will explain their merits. They 



Fig. 321. 



are manufactured of common pottery- 

 ware, and are each about 15 inches long, 

 and 5 inches wide, with a concave bottom 

 to fit upon 4-inch water-pipes ; they are 

 placed in various parts of the houses. 

 When filled with water, they continue to 

 give out a genial and uniform vapour, 

 and of course are not liable to rust, or 

 any of the objections raised against the 

 metallic pans usually employed for this 

 purpose. The only fault we have to 

 those above noted is, that they are 

 rather too small ; but this is a fault 

 readily remedied, or a greater number 

 of them may be employed. Supplying 

 humidity to the atmosphere of a hothouse 

 is much better effected by the use of 



