FRINCIPLES OF VENTILATION. 257 



judgment upon broader and safer principles ; and, in order to 

 reach this point, let us briefly consider the nature of atmospheric 

 action upon hot-houses. 



Before entering upon any illustration of its practical effects 

 upon these structures, we will give an extract from Dal- 

 ton's Chemical Philosophy, which will enable us to account 

 more clearly for some of those results that we have often 

 observed, and which have so often humiliated our practical pride 

 and baffled all our boasted experience. In fact, they have been 

 considered as belonging to that class of unaccountabilities which 

 our Creator has placed beyond the ken of human discovery. 



" It is a remarkable fact," Dahon observes, " and has never I 

 believe been fully or satisfactorily accounted for, that the atmos- 

 phere, in all places and seasons, has been found to decrease in 

 temperature as we ascend, and nearly in arithmetical progression. 

 Sometimes this fact may have been otherwise, i. e., that the air 

 was colder at the surface of the earth than above; particularly at 

 the breaking up of a frost, I have observed it so. But this is evi- 

 dently the effect of a great and extraordinary commotion in the 

 atmosphere, and is generally of very short duration. What, 

 then, is the occasion of this diminution of temperature in ascend- 

 ing? Before this question can be solved, it may be necessary 

 to consider the defects of the common solution. Air, it is said, 

 is not heated by the direct rays of the sun, which passes through 

 it as a transparent medium, without producing any calorific 

 effect till they reach the surface of the earth. The earth, being 

 heated, communicates a portion to the atmosphere, while the 

 upper strata, in proportion as they are more remote, receive les« 

 heat, forming a gradation of temperature similar to what takes 

 place along a bar of iron, when one of its ends is heated." The 

 first part of the above solution is probably correct. Air, it would 

 seem, is singular in regard to heat ; it neither receives nor dis- 

 charges it, in a radiant state. If S9, the propagation of heat 

 through air must be opposed by its conducting power, the same 

 as in water. Now, we know that heat, applied to the under sur- 

 face of a column of water, is propagated upward with great 

 velocity, by the actual ascent of the heated particles ; it is equally 

 rertain that heated air ascends in the same way. From these 



