POLMAISE METHOD OF HEATING HOT-HOUSES. 69 



to bear upon forms of boilers and situations of pipes, how absurd 

 it would be to suppose that we have got at all beyond the 

 threshold of Polmaise heating; and when we consider what has 

 been accomplished with a system wholly unnatural, and partially 

 unsound, what may we not expect to achieve with a mode of 

 heating where the principles defy attack. But we shall cer- 

 tainly find our error if we suppose that the success of Polmaise 

 is independent of all conditions. There is no question that heat 

 causes currents when applied to liquids ; that such currents will 

 cause an actual circulation ; but conditions are necessary to this 

 before the principle can become useful ; for, provided the boiler 

 is situated above the return-pipe, the circulation will not take 

 place unless express arrangements have been made with the flow. 

 And numerous have been the failures, partial or total, of hot- 

 water apparatus from the relative position of the return-pipe 

 and boiler; and it is, I believe, pretty generally understood, 

 that a rise in the flow-pipe, and a fall towards the boiler in the 

 return, is the best mode of obtaining a good hot-water circula- 

 tion. May we not expect, then, that certain conditions will 

 equally attach to render Polmaise effectual or otherwise ; and 

 already, to my knowledge, two failures have occurred, — one at 

 least partial, and one entire, — and both apparently caused by the 

 situation of the cold-air drains, which are the return-pipes of 

 Polmaise. In the case of total failure, the cold-air main drain 

 rises a foot before entering the chamber, and of course the air, 

 not having yet become warmed, cannot rise, but lies dead in the 

 drain, and the circulation is effectually impeded. And these 

 failures promise to be more instructive than even the success of 

 the house at Nutfield ; for there the main drain runs on a level 

 into the chamber, as shown in longitudinal section, and even 

 after its arrival in the hot chamber it ascends before reaching the 

 iron plate, which may be regarded as the air-boiler. Surely, 

 well as it acts, there is every reason to think its operation would 

 be far more effectual provided the cold-air drains had a gradual 

 fall, say of 1 in 30, towards the chamber, and if the stove Mas 

 lowered in such a manner that the top of the plate should be 

 level with the bottom of the drain, that the air might not have 

 to make any ascent till it received the full impetus arising from 

 expansion. The best form of stove, the most desirable position 

 in which to place it, the relative size of drains, and their proper 

 inclination, — for the solution of all these points we must quietly 

 wait for accumulated experience ; and as at the commencement 

 of this paper I alluded to the importance which the subject of 

 the diffusion of artificial heat assumes in this country, so at its 

 close I would again venture to urge it upon your Society. How- 

 ever much horticulturists may be indebted to your Society for 



