OCTOBER, 1902. 
migratory motion. The following discussion will c'cal in the main 
with such of these problems as can be solved by means of the 
telescope supplemented by field observations. 
Numbers Associated in Migration. — It has been known for a 
long time that migrations occur both during the day time and 
during the night, but it is only within comparatively recent years 
that a fair conception of the extent to which the nocturnal migra- 
tions take place has been attained. Prof. A. S. Flint of the 
Washburn Observatory has calculated from data collected by Dr. 
O. G. Libby in 1897, that nine thousand birds per hour passed 
within the arc of a circle subtended by the angle through which 
the telescope passed during the entire period of observation. This 
gives us a fair mathematical estimate of the magnitude of the 
nocturnal movement. 
The Altitude Attained in Migration. — Estimates of the alti- 
tude of migration as obtained by means of the telescope show that 
former investigators have placed their estimates far too high. In 
the observations taken at Beloit from 2 :oo to 3 130 a. m. the tele- 
scope made an average angle of about fifteen degrees with the 
horizon. The telescope at this time was about one mile from 
that portion of the river over which the birds were observed. This 
means that the majority of them were not over fifteen hundred 
feet above the surface of the earth. 
Estimates from data collected at Detroit show the flight to have 
been slightly his/her. but still far below the estimates of former 
investigators. Mr. H. S. Warren writes from that place as fol- 
lows : "The observations were taken at the Cartwright observa- 
tory located about three-fifths of a mile from the river. The 
telescope, a 6-inch refracting instrument, equatorially mounted 
with an 8-inch focus, was at an average angle during the period 
of observation, of about thirty degrees. If the birds that were 
traveling northeast, which numbered considerably more than one- 
half of all noted, were following the course of the river, w^hich 
seemed evident, they must have been somewhat more than one- 
half mile above the surface." 
The telescopic observations show that there may be a zone of 
considerable depth, birds choosing variable altitudes in which 
to perform their migrations, but by far the greater number do 
not attain an altitude much over one-half mile from the earth's 
surface. 
Associations of Individuals and Species. — A more detailed 
consideration of the telescopic observations collected for this thesis 
tends to establish, with some modifications, the results obtained by 
