PRINCIPLES OF VENTILATION. 259 



feature, in its regard to heat. We see, in a perpendicular col- 

 umn of air, a body without any change of form, slowly and 

 gradually changing its capacity for heat, from a less to a greater ; 

 but all other bodies retain a uniform capacity throughout their 

 substance. If it be asked why an equilibrium of heat should 

 turn upon the quality in quantity, rather than in temperature, I 

 answer, I do not know ; but I rest the proof of it upon the fact 

 of the inequality of temperature observed in the atmosphere in 

 ascending, which invariably becomes colder as we ascend in 

 height ; while, in artificial atmospheres, as in the case of a hot- 

 house, the fact is quite the reverse. If the natural tendency of 

 air was to an equality of temperature, there does not appear to 

 me any reason why the lower regions of air are warmer than 

 the higher, or why the law of equalization held good in one case 

 and not in another. 



To enable us to apply these arguments more clearly to our 

 subject, it will be necessary more fully to consider the relation 

 of the atmosphere in regard to heat ; and the arguments already 

 advanced in behalf of the principle we are endeavoring to estab- 

 lish, are powerfully corroborated by the following facts. 



We find, by the observations of Bougeur, Sassure, and Gay 

 Lussac, that the temperature of the atmosphere, at an elevation 

 where the weight is half that at the surface, (about 14,000 feet, 

 or less than three miles,) is reduced in temperature 50 Fahren- 

 heit; and, from experiment, it appears that air, suddenly rarefied 

 from two to one, produces 50 of cold. Hence we might infer 

 that the stratum of air at the earth's surface being taken up to 

 the height above mentioned, preserving its original temperature 

 and suffered to expand, becomes two measures, and is reduced to 

 the temperature of the surrounding air, and vice versa. In like 

 manner, we may infer, if a column of air from the higher strata 

 of the atmosphere were condensed and brought into a horizon- 

 tal position on the earth's surface, it would become of the same 

 density and temperature as the air around it, without receiving 

 or parting with any heat whatever. Another important argu- 

 ment in favor of the theory here advanced, may be derived from 

 the contemplation of an atmosphere of vapor. Suppose the pres- 

 ent aerial atmosphere were to be substituted for one of aqueous 



