17 



migration to atmospheric cold and warm waves is discussed, and the 

 statement is there made that the warm waves begin in the northwest 

 and move toward the southeast. The cause of this lies in the move- 

 ment of the area of low pressure. It is a law of the movement of winds 

 that they go toward an area of low pressure, and from an area of high 

 pressure. If, then, an area of low pressure develops, say in southwestern 

 Dakota, it will be but a few hours before a south or southeast wind will 

 be blowing over Nebraska and Kansas, and a warm wave will be started 

 in these States. As this area passes eastward to Minnesota its effect 

 will begin to be felt in Iowa, Missouri, and Arkansas, while by the time 

 it has reached Lake Michigan it will probably have produced southeast 

 winds even to the Gulf of Mexico. But an area of low pressure is fol- 

 lowed by one of high pressure, producing an opposite effect, and the 

 isotherms which bent north to welcome the coming of the low area turn 

 rapidly southward before the icy breath which blows from an area of 

 high pressure. Thus the cold and w r arm waves both come from the 

 same quarter, and both move in the same direction ; that is, the direc- 

 tion in which the area of low pressure is advancing. Since it is known 

 that low pressure is generally accompanied by clouds and rain, while 

 areas of high pressure are cloudless, it would be naturally supposed 

 that migration would take place during high pressure ; but, as has 

 already been stated, the area of low pressure attracts a south wind, and 

 the increased warmth more than overbalances the cloudiness. Fully 60 

 per cent, of the spring migration of 1884 took place in cloudy weather. 

 It is probable, though I am not aware that it has as yet been proved, 

 that in the fall migration the reverse is the case, and the larger move- 

 ment takes place in clear weather. 



Following is a full record of the relation of migration to atmospheric 

 conditions for the seven days from March 19 to 25, 1884, contrasted with 

 a week's migration in May. 



March 18, 1884, at 11 p. m., there was no marked atmospheric dis- 

 turbance throughout the United States. The minimum of the cold 

 wave had occurred the day before and the temperature was gradually 

 rising in the Upper Mississippi Valley. It is this part of the Mississippi 

 Valley (from latitude 39° northward) to which the present study is con- 

 fined. The temperature was quite high (50° at Saint Louis, and 37° at 

 Saint Paul), but fell rapidly from Saint Paul northward till it reached 

 20° at Moorhead, Minn.* The barometer varied only two-tenths of an 

 inch from 29.9 inches in eastern Arkansas and southern Illinois to 30.1 

 at Moorhead, Minn. The prevailing winds were very light E. to N". 

 The weather was cloudy, with several light rains. There was little 

 change toward the morning of March 19, except the shifting of the wind 

 to K. and NW,, while the area of low pressure moved east to Cape 

 Hatteras. Very little migration took place, and the few birds that 



*It need hardly be said that no one can follow these remarks intelligently without 

 referring to the map accompanying this report. 



7365— Bull. 2 2 



