HEATING BY HOT WATER, HOT AIR, AND STEAM. 171 



We find, by experience, that no system of heating horticul- 

 tural buildings in all respects answers the purpose so well as a 

 hot-water apparatus, well constructed, and judiciously arranged, 

 in regard to the amount of work it has to do, so that it may not 

 be necessary to strain it, on exigencies, to its maximum point of 

 strength. In whatever point of view it may be regarded, it is, 

 undoubtedly, the best for all practical purposes ; and the best 

 possible evidence of its utility is derived from the fact, that no 

 case has ever come under our knowledge, wherein it has failed 

 to give complete satisfaction, when it has been properly con- 

 structed, rightly managed, and judiciously arranged, in regard 

 to supplying a sufficient amount of radiating surface for the 

 work it has to do. 



3. Comparison of hot air with hot water, as a mode of heating 

 horticultural structures. Various erroneous opinions and prin- 

 ciples have been theoretically and practically promulgated, in 

 regard to hot-air heating ; and, carrying with them, in general, 

 some degree of plausibility, and in some cases emanating from 

 men of learning, have led many, who have not studied the mat- 

 ter attentively, into very great errors. However invidious, 

 therefore, may be the task of pointing out such errors, we con- 

 sider it our duty, when treating on the subject at large, not only 

 to exhibit what we consider to be the true principles, but to 

 show where erroneous principles have been adopted. This must 

 serve as an apology for the freedom with which the advocates 

 of Polmaise, and other methods of hot-air heating, and the sys- 

 tems they approve, are descanted on in this section. 



We have already observed that the cooling of a heated body, 

 under ordinary circumstances, is evidently the combined effect 

 of radiation and conduction. The conductive power of the air 

 is principally owing to the mobility of its particles, for, otherwise, 

 it is one of the worst conductors we are acquainted with. 



Atmospheric air, in passing into a house over a highly heated 

 surface, must necessarily lose a large quantity of its contained 

 moisture ; [see 1. Effects of Artificial Heat, of the preceding 

 section ;] and, as its capacity for taking up moisture is increased 

 according to its temperature, it follows that a great demand 

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