100 BULLETIN OP THE 



(ABSTRACT.) 



Mr. Abbe called attention to a recent memoir by Professor 

 Loomis, of Yale College, on the storms of tlie United States 

 during the years 1872 and 1873. He stated that tlie generaliza- 

 tions announced by Professor Loomis constituted a most im- 

 portant contribution to our knowledge of this subject, and that 

 the laws mathematically deduced from mechanical principles by 

 Mr. Wm. Ferrel in his memoir on Winds and Currents, were 

 in every instance corroborated by observations in both Europe 

 and America. He stated that the law announced by himself 

 at Cincinnati in 1869 remained abundantly conilrmed by daily 

 experience, and might be concisely expressed as follows: a storm 

 centre moves toward the region where a given barometric or other 

 condition produces the greatest precipitation of aqueous vapor, or 

 in which the latter is most rapidly taking place. In explanation 

 of this law he 'added that when we take into consideration all the 

 causes that contribute to produce the precipitation of vapor 

 (whether in the form of haze, fog, cloud, rain, snow, or hail)^ 

 we are able to account with great accuracy for the direction and 

 velocity of movement of areas of low barometer. 



Atmospheric precipitation is produced by cooling the air, and 

 a fall of temperature in any gas is the consequence either of the 

 radiation of heat or of the absorption of heat in performing in- 

 ternal work. 



The mechanical absorption of heat is an important feature when- 

 ever masses of air are elevated and allowed to expand. This 

 occurs under the following circumstances : (1) whenever strong 

 winds blow, and in consequence of the inertia of the air and the 

 friction on the earth's surface, produce vertical currents ; (2) in 

 consequence of winds being pushed up an inclined plane, such as 

 the great plains of the Mississippi Basin, or the ascents on the 

 east and west sides respectively of the Appalachian range, and 

 the Sierra Nevada ; (3) in consequence of the elevation of masses 

 of warm air above the masses of cold air, which latter flow, for 

 example, from the extreme N. W. southward to the Gulf of Mex- 

 ico, down the gentle grade of the Missouri and Mississippi Val- 

 leys (under this head are included the formation of the local 

 thunder storms). 



The radiation of heat may take place either (1) outward into 

 space whenever the air above is dry; or, (2) downward either to 

 the cold earth, or to rnasses of cold air that have undei-run and 

 uplifted warmer layers: radiation into space is especially effec- 

 tive after a mass of moist air has been thus uplifted. 



The radiation of heat and the visible precipitation of vapor are 

 remaikably counteracted whenever extensive fires prevail, by the 

 presence in the atmosphere of very minute particles of carbon 

 or of vegetable ashes, which have the property of attracting about 

 themselves quite dense atmospheres of aqueous vapor precisely 

 as oxygen is occluded by finely divided platinum. 



