108 THE FLIGHT OF BIRDS 
apart ; they run at right angles to the usual stream 
of autumn migrationWhich flows from NN.E. to 
SS.W. He takes up his position at a point in one 
of these lines; at the corresponding point in the 
other line his assistant is stationed. With a stop- 
watch he marks the moment at which a bird passes 
his post of observation: at the moment it passes the 
other post, the assistant telephones the fact to Dr. 
Thienemann, who records it on his stop-watch. The 
direction of the wind is taken and the exact angle 
it makes with the bird’s line of flight ; its pace is 
measured by means of an anemometer. With these 
data, whatever the angle may be, it is not difficult 
to calculate the distance travelled by the wind with 
the bird or in the opposite direction, while he is 
covering the measured half kilometre. After finding 
the rate at which the bird travels per second, per 
minute, per hour, Dr. Thienemann subtracts or adds 
the distance travelled by the wind, with or against, 
during the same time. Thus the disturbing wind- 
factor is eliminated and he arrives at the bird’s own 
velocity. All this is wonderfully ingenious and 
wonderfully thorough ; the results have great value 
because they are undoubtedly dependable. We can 
only regret that the species of birds observed do not 
include any of those observed by Commander Lynes. 
Moreover it seems probable that they were not flying 
at fullspeed. Migration, as a rule, goes on at a great 
height ; these birds had descended to a low level, 
and with land beneath them were flying probably 
in leisurely style. The much greater velocity of 
Commander Lynes’s Quailsis accounted for, I believe, 
if we bear two facts in mind ; they are, presumably, 
