METEOROLOGY. 349 



cast and iiortbeast and cause heavy rainfall. The Boeu is thus pre- 

 sented as a whirl around a horizontal axis. Koppen states that, since 

 his first memoir in 1879, a slight change in his views has taken place, iu 

 that although he retains the important feature of the descent of cool 

 air driven down by its density and falling rain-drops, yet he does not 

 give so great an importance to this falling air as he did before. 



In general, on the southeast side of any barometric depression, inso- 

 lation causes a very warm region and at the same time, within a few 

 hundred meters of the earth's surface, a subsidiary depression, although 

 iu theatmospliere immediately above this the isobars retain their eUip- 

 tical form. The eastern portion of this region is thus withdrawn from 

 the influence of the cooling current on the west side, and as a feeble 

 southeast current increases in temperature. The western part, on 

 the other hand, is early overflowed by the cool air from the west, 

 which, by reason of the progressive movement of the i»rincipal depres- 

 sion, always comes from a more northerly point. By the pushing up 

 of the warmer air rain is caused on this limit of the warm and cool 

 region. By this, lower temperatures are caused close to the limit of high 

 temperatures, and a temperature gradient produced which, following 

 both'the general current of air and the perpetually renewed precipita- 

 tion on the temporary edge of the warm region, moves toward the side 

 of the higher temperature. By this temperature gradient combined with 

 the conditions of motion a pressure gradient is produced in the lower 

 600 meters of air which propagates itself in the same direction. This 

 pressure gradient (favored by the fact that the remaining or horizontal 

 distribution of pressure both above and below causes a current that 

 flows approximately with that due to the steep gradient) causes an ex- 

 traordinarily strong wind along the whole breadth of the region during 

 its passage over, which usually lasts about twenty minutes. This stormy 

 portion of the current is about two-thirds due to the air dragged by the 

 downfalling mass of rain, while in front of it the air is ascending. 

 Special places in this stormy region, which generally crosses the south- 

 west current almost perpendicularly, are distinguished by special inten- 

 sity of the storm, and other portions by a formation of hail. There thus 

 originate regions of hail storm and of destruction, as evidences of the 

 passage of the storm, and whose greatest diameters are approximately 

 perpendicular to the greatest temporary extension of the Boen. So 

 long as the thunder-storm moves against the sun it is subject to the 

 influence of a daily period, since it originates in the morning, is best 

 develoiied wherever it may be at the warmest time of day, and then 

 with the beginning of the night rapidly diminishes in strength and soon 

 disappears. {Z. 0. G.M., xix, p. 12.) 



300. [If we properly appreciate the points made by Dr. Koppen in 

 his beautiful study, we should say that that which is thoroughly novel 

 is the bringing out so clearly the fact that when once the development 

 of the thunder cloud has reached the point of decided rainfall, and with 



