Transactions of the Horticultural Society. 



183 



which, having stood the test of experience, and being far beyond the bounds 

 of common practice, afford a proof of the benefit of studying first principles 

 in new and untried cases ; but, in general, for hot-houses, twice the num- 

 ber of feet contained in the area of the surface of glass will be equal to the 

 number of cubic feet of air which that surface should heat per minute when 

 in full action. 



" 15. Now the heat given off by the surfaces of the apparatus depends 

 on the kind of materials they consist of, and their temperatures. The fol- 

 lowing table shows the boiling point and temperature of the heating sur- 

 face for different liquids when confined by iron or glass ; also their specific 

 heat, or that quantity of heat they can convey, when that conveyed by an 

 equal volume of water is unity. 



" 16. If the cubic feet of air to be heated per minute be multiplied by 

 the number of degrees it is to be warmed, and the result be divided by 

 twice the difference between the temperature of the house and that of the 

 surface of the pipes, the result will be the feet of surface of iron pipe, &c., 

 required. Thus, if 1000 cubic feet per minute are to be warmed, and the 

 extreme case is supposed to be, that when the external air is 20° the 

 house should be 50*^ ; and, therefore, the air is to be warmed 30° ; and with 

 water the surface will be 190° when the water boils *, but only 180° in the 

 average state ; therefore. 



1000 X 30 



30000 



2(180 — 50) ~ 260 

 If we employ brine for the same case ; 

 IOOOX.30 



= 1 1 G feet of surface^ 



then. 



= 106 feet. 



2(192 — 50) 

 And, with oil, 



1000X30 P ,, 



^TTYQ^^rQX = 32i ft. would answer the purpose. 



When bright tinned iron, earthenware, &c., are employed for pipes, much 

 more surface is necessary. 



" The advantage of using a fluid which bears a high temperature without 

 boiling is therefore considerable in reducing the quantity of surface re- 

 quired to produce a given effect ; oil requiring only one third of the surface 



* The mean and extreme temperatures for the neighbourhood of Lon- 

 don may be obtained from the oh-icrvatious of Mr. Daniell, in his Meteoro- 

 logical Essays, 



N 4 



