INTRODUCTION. XXXV 



with the air, making this ascending column a true column of 

 heated air, that is to say, of a lighter gas ; for the weight of the 

 water which passes into the liquid state, is far from compensating 

 the excess of levity which proceeds from the more elevated tem- 

 perature which the air preserves. (This weight only equals one 

 fifth of the diminution of the weight in ordinary circumstances.) 



Thus, the higher the column is, the greater is the ascending force, 

 and the rushing in of the surrounding air on all sides will be pro- 

 duced with more energy. To understand this effect better, let us 

 consider a mass of warm and dry air rising in the midst of a 

 colder atmosphere. In proportion as this air rises, it will expand, 

 because of the less pressure which it will experience, and conse- 

 quently become colder ; it will arrive then quickly at an equili- 

 brium both of temperature and pressure with a layer more or less 

 elevated, which it will soon reach, and in which it will remain ; 

 but if this only cause of cold, expansion, is overbalanced by a 

 cause of heat, for example the heat furnished by the vapor which 

 is condensing, this air will remain constantly warmer than would 

 have been necessary to attain the same temperature and pressure 

 as the surrounding air. It will then be constantly lighter, and the 

 higher the column, the greater the ascending force. 



The calculations of Mr. Espy show, without the slightest doubt, 

 that the column of damp air regaining in temperature, by the 

 condensing of the vapor, a part of the heat lost by expansion ; 

 this column always remains warmer than the air which is at the 

 same height with each of its parts. Finally, Mr. Espy furnishes 

 the exact data which are still wanting to science, by the experi- 

 ments made upon the temperature which the air preserves by the 

 effect of condensation of the vapor in a closed vessel which he 

 calls a " nephelescope," and in which he compares the thermo- 

 metrical fall produced in the air by a diminution of superincum- 

 bent pressure, to what takes place in nature, whether operating on 

 dry, or employing damp, air. Notwithstanding the influence of 

 the sides of the vessel, every time a light cloud is formed in the 

 apparatus, the temperature undergoes a much less reduction than 

 that which takes place when the point of precipitation of vapor 

 has not been attained, or when the experiment is tried on dry air. 



The theory of Mr. Espy also accounts very well for the forma- 

 tion of a true cloud analogous to the cumulus with horizontal base, 

 from the moment when the warm and damp air has acquired such 

 an expansion, that the cold produced by it will cause a precipita- 

 tion of water, and the base of the central cloud of the tornado, if 

 it is horizontal, as is the case in the great meteors of this nature, 

 should be lowered in proportion as the moist air which is carried 

 up is more fully charged with vapor ; this base, like that of the 

 cumulus, being of necessity found at the point where the temper- 



