35 



which first led to the adoption of the plan of laying down the phenomena 

 of storms on maps, as here recommended to you : 



1st. When the air near the surface of the earth in a particular locahty 

 acquires a higher temperature or a higher dew point than that of surround- 

 ing regions, it will ascend, on account of a less specific gravity — perpen- 

 dicularly if there is a caim. 



2d. As it ascends it comes under less pressure and expands, and be- 

 comes colder by expansion, about one degree and a quarter ior each hun- 

 dred yards. 



3cl. The dew-point falls about one quarter of a degree, by the expansion 

 of the vapor, for each hiaidred yards; and, therefore, when the ascending 

 air rises as many hundred yards as the dew-point is below the temperature 

 of the air at the surface of the earth in degrees of Fahrenheit, the cold of 

 expansion of the air will begin to condense the vapor contained in it, and 

 thus form cloud. 



4th. As soon as the vapor begins to condense into cloud, the latent 

 caloric will begin to be evolved, which will diminish the cooling effect of 

 its expansion in its further ascent, sometimes a little more and sometimes 

 a little less than one-half, according as the dew-point is above or below 

 70° of Fahrenheit. 



5th. When the air has ascended high enough to have condensed by 

 the cold of expansion, from diminished pressure, one-hundredth of its 

 weight of vapor, its temperature will be about 48° warmer from the evolu- 

 tion of the latent caloric than it would be by going up to the same height 

 without vapor in it, and will then be about 48° warmer than the air around 

 the cloud at the same elevation, and of course about one tenth hghter. 



6th. The air in the cloud being thus specifically much lighter than the 

 surrounding air, will ascend, and, in ascending, spread out in all direc- 

 tions above, overlapping the air of surrounding regions, and thus causing 

 the barometer to rise all round the cloud to a distance proportional to the 

 magnitude of the cloud. 



7th. Whilst the barometer is rising all round the cloud from the in- 

 creasing weight of the air, it will fall under the central regions of the cloud 

 in proportion to the quantity of air spreading out above. 



8th. The air near the surface of the earth will now rush in on all sides 

 from the regions around the cloud, where the barometer stands high, to- 

 wards the central regions, where the barometer stands low, with a volocity 

 proportional to the square root of the difference of pressure. 



9th. The air thus rushing in under the cloud on all sides will ascend, 

 and carry up its vapor with it, and will condense it into cloud by the cold 



