16 



Atlantic coast traverse great distances, sometimes covering from 300 to 

 600 miles at a single flight ; and it cannot be said positively that the 

 larger birds do not do the same over the land. Still, the records so far 

 made seem to indicate that the smaller land birds, such as Warblers, 

 Finches, and the like, do not perform long journeys at one time when 

 over land, but their voyages over the Gulf of Mexico prove that even 

 these small species possess great power of flight. 



The preceding discussion shows that we are entirely wanting in exact 

 knowledge on the subject, and that for some time to come all we can 

 hope to do is to gather material. In this connection the facts accumu- 

 lated by the keepers of light-houses are of the utmost importance, and 

 the publication of the report containing these data is looked for with 

 great interest. 



THE RELATION OF MIGRATION TO BAROMETRIC PRESSURE AND TEM- 

 PERATURE. 



In studying the spring migration of 1884 use was made of the tri-daily 

 weather reports of the Signal Service, of which about 50 stations were 

 in or near the Mississippi Valley. These reports give the temperature, 

 state of barometer, dew point, direction and force of wind, amount of 

 rainfall, and character of the sky. The observations on which they are 

 based were taken at 7 a. m., 3 p. m., and 11 p. m. In the following study 

 the 11 p. m. records were used for those birds which migrate by night, 

 and the 7 a. m. records for such species as Ducks and Geese, which per- 

 form the bulk of their movements in the forenoon. 



To render the study more easy, weather maps were made, one for each 

 day, based on the 11 p. m. observations. The maps were made as nearly 

 as possible like those now printed daily by the Signal Service at Wash- 

 ington; that is, the state of the weather and the direction of the wind 

 were marked at each station on the map in symbols which are plain and 

 easily comprehended, so that the eye could take in at a glance the gen- 

 eral state of the weather in the whole Mississippi Valley. At each 

 station the temperature, state of barometer, and force of wind were indi* 

 cated. Dotted lines were then drawn connecting all places having the 

 same temperature, and solid lines connecting places of equal baromet- 

 ric pressure. The former, called isothermal lines, were drawn for every 

 five degrees of Fahrenheit; while the latter, called isobaric lines, were 

 drawn for every tenth of an inch of pressure. The area of the lowest 

 pressure is never stationary, but is constantly moving, and in an east- 

 erly direction. It may be moving northeast, east, or southeast, and 

 rarely north or south; but never northwest, west, nor southwest. The 

 usual direction in the Mississippi Valley is a little south of east. It so 

 happens that the particular wave which we study moves northeast- 

 ward, but this is an exception to the general rule. Though it may 

 move south or north for a time, it will surely turn east in the end. 

 In the body of the report, under the Purple Martin, the relation of 



