19 



change was at this time slightly felt. The main portion came from the 

 same districts as on the day previous. There was practically no ad- 

 vance of the van, but a filling up of the country already traversed by 

 the scouts. 



On the night of March 21, at 11 p. m., an area of low barometer (29.9 

 inches) was passing eastward across the Upper Mississippi Valley and 

 was central at North Platte, Eebr. It produced SE. winds of moderate 

 strength in all the Mississippi Valley except the extreme northern part, 

 above La Crosse. The temperature rose throughout the district to 50° 

 at Saint Louis and 35° at Saint Paul ; but north of this, beyond the in- 

 fluence of the SE. wind, it fell rapidly to 18° at Moorhead and 11° at 

 Saint Vincent. The isotherm of 40° was carried up to La Crosse. As 

 morning approached, the temperature still rose iu the northern part 

 and the sky became overcast with some local clouds and rain. It was 

 a night of much migration, owing to the influence of the area of low 

 pressure, which at 7 a. in., March 22, was central at Omaha and Yank- 

 ton, where the barometer at 3 rj. m. registered 29.71 inches. This was 

 a fall at Yankton of .32 inch during sixteen hours, while the center of 

 the low area moved but a few miles ; and the necessary result was a 

 great rise in temperature and consequently great movements among 

 birds. But the movements took place only to the east of the low press- 

 ure area ; for it is a law of atmospheric circulation that the winds are 

 attracted from the south, not directly toward the center of the low press- 

 ure area, but toward places to the east of ifc in the same direction that 

 it is moving, while the winds which it attracts from the north move 

 toward places to the west or behind it. Migration, therefore, would be 

 looked for in vain to the south, west, or north of Y r ankton. The whole 

 of this immense movement, which iu number of records was as great as 

 that of the three previous days together, and in number of individuals 

 was many times greater, took place to the southeast and east of Yank- 

 ton. The SE. winds prevailed up to La Crosse, and this place also 

 marked the limit of the night's movement in that direction. A map 

 was made of the migration which took place on this day, and it was 

 found to cover a very nearly circular area, 250 miles in diameter, the 

 center of which was midway between Keokuk and Davenport. Some 

 idea of the great number of birds which were migrating during this night 

 may be had from the fact that at Saint Louis twenty-six different species 

 were noted as having arrived or increased. It is well to bear in mind 

 that all these birds were migrating on a rapidly falling barometer, hence 

 in the face of what is usually considered a sign of an approaching storm ; 

 and it may be noted also that all this great movement did not advance 

 the van, which remained where it had been before. 



March 23, bird migration was at a standstill. The area of low press- 

 ure, which was central at Yankton in the afternoon of the 22d, had 

 moved to Saint Paul by 11 p. m., the barometer falling steadily to 29.61 

 inches. During the night it moved NE. to Marquette, Mich., falling 



