POLMAISE METHOD OF HEATING HOT-HOUSES. 71 



tention must be drawn is, that the heated air, acting upon the 

 slates forming the bottom of the bed, will of necessity exert 

 a drying influence on whatever rests upon them ; and that, sup- 

 posing this to be soil in which plants were bedded out, this 

 would become extremely dry and ungenial : the roots which 

 found their way downward would soon perish. Exactly the 

 same difficulty arises where the bottom heat is communicated by 

 hot-water pipes running beneath the plunging-bed, namely, their 

 parching effect on the superincumbent soil ; while, if open tanks 

 are employed, then the other extreme is incurred, and the soil is 

 soured by incessant steam. It has been suggested to me that 

 this difficulty is best overcome by making the bottom of the bed 

 water-tight, and placing on the two or three inches of broken 

 brick, rubbish or pebbles, down among which water can be 

 poured in different places, which will diffuse itself and become 

 absorbed, and gradually moisten the soil above it as it is con- 

 verted into vapour by the heat below, while any excess can be let 

 off by having holes at the requisite level fitted with plugs. All 

 these points will be made more or less perfect according as they 

 fall into the hands of ingenious men or otherwise. I only wish 

 to draw attention to them as affecting the good practical working 

 of the system, leaving others to make the application which their 

 own peculiar and individual case may require. The only other 

 subject I wish to notice is the form of the top of the stove, espe- 

 cially as connected with gaseous exhalation : with plates formed 

 as mine are, there must always exist a liability to this, and on 

 one or two occasions I have clearly detected the presence of gas 

 slightly in the house — when there has been fresh fuel supplied 

 and the damper pushed in so as to stop the chimney (thus 

 forcing its escape, as it were) — now though very slight, and only 

 occurring when forced to escape, I think it desirable so to con- 

 struct the plating that even this shall be prevented, which may 

 be effected by having the plates cast in one, with a projecting 

 rim on the under surface sinking into a sand-groove. However, 

 it is with much pleasure that I take this opportunity of stating 

 that a form of plating has been prepared and shown to me by a 

 well-known and very eminent firm largely engaged in the horti- 

 cultural trade, which I consider far superior to the flat plating I 

 have employed : it is in several pieces to allow of expansion, and 

 yet arrangements are provided which are calculated to prevent 

 the possibility of exhalation, and at the same time in the 

 same area a much larger surface is exposed to absorb heat from 

 the fuel and communicate it to the air ; and, should it accom- 

 plish these points, I am sure it will meet with every encourage- 

 ment it deserves. 

 Bee. 10, 1846. 



