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 warm waves both come frem 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 fall 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. Itis 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 tili 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 N. and NW., while the area of low pressure moved east to Cape 
Hatteras. Very little migration took place, and the few birds that 
*Tt need hardly be said that no one can follow these remarks intelligently without 
referring to the map accompanying this report. 
7365—Bull. 2 2 
