POLMAISE METHOD OF HEATING HOT-HOUSES. 



65 



heated surface of the stove is the coldest air in the house, or 

 otherwise it would not be upon the floor ; and when I notice 

 certain facts observed by Mr. Murray at Polmaise, and by myself 

 at Nutfield, it will be evident that the result is exactly what we 

 might anticipate from using a power, not only able, but forced to 

 move at the extreme of its speed, namely, a closer approximation 

 to uniformity of temperature. 



Another advantage resulting from the employment of Pol- 

 maise is, that not only is uniformity of distribution secured, but 

 this uniformity is independent of external circumstances. Many 

 persons, mistrusting anything new, though unable to deny results 

 that are evident to all who choose to verify them, exclaim, that 

 it is all very well now, but how will this plan keep out the frost 

 in the winter ? — while those who have so certainly foretold its 

 complete failure look forward to the winter frost as their forlorn 

 hope. But when we examine principles, we find that the velo- 

 city of the current between the two buildings is in proportion to 

 the difference of the temperature of the air in the two. It is 

 evident, therefore, that the only effect exerted on the air within 

 the hot-house by the cooling influence from without will be to 

 send it more rapidly to the source of heat ; so that, provided a 

 proportionate amount of fuel can be burnt, and caloric evolved 

 in the chamber, the air of the house will take care that the heat 

 so evolved is distributed. It will no longer be necessary, in 

 order to keep the frost out of one end of the hot-house, to par- 

 roast the other, as in the old flue system ; for while in all sys- 

 tems of heating this same principle operates to check the varia- 

 tion to a certain extent, in no system can it act so efficiently as 

 in that in which the coldest particles go first to be heated, thus 

 insuring the greatest possible amount of velocity of distribution. 



The last advantage I shall notice as pertaining to Polmaise 

 heating is, the necessary and constant motion of the atmosphere 

 which it secures ; and this will probably be found by horticultu- 

 rists to be one of the most important considerations affecting the 

 question. My knowledge of vegetable physiology is not suffi- 

 cient to enable me to prosecute this consideration far, and those 

 who understand the subject best are not unanimous in their opi- 

 nion as to what are the precise effects of atmospheric motion 

 upon vegetation. Whether it has any other effect than that of 

 bringing fresh supplies of carbonic acid to be acted upon by the 

 foliage or not, it must be evident to the most casual observer 

 that some important results proceed from this cause. We have 

 occasionally very high temperatures externally both by day and 

 night, yet we never see that peculiar appearance called " draw- 

 ing" among plants grown in the open air. Is not a stagnant 

 atmosphere the cause therefore, rather than the amount of tem- 



VOL. II. p 



