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 at7a.m.,3 p.m.,and11p.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 
saine 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 
a 
