494 
Bulletin Xo. 162.—1915. 
probable time of laying of the second egg. This formula is based on 
all the available data, and is not corrected for the different months. 
For convenience of reference the more important* constants which 
have been developed in the foregoing section are here brought 
together. 
Mean time of laying of A.5:10 P. M. 
Mean time of laying of B (on second day following A) 1:22 P. M. 
Standard deviation of time of laying of A. . 1.008 + 0.048 hours. 
Standard deviation of time of laying of B . . 1.002 yb 0.048 hours. 
Mean interval between laying of A and B . 44.21 hours. 
Standard deviation of same . 1.120 yfy 0.079 hours. 
Coefficient of correlation of laying of A and B . 0.38 yb 0.057. 
Regression straight line of same. y — — 0.48 + 0.378x. 
Time of Hatching of Eggs A and B. 
Aristotle (1910, 562 b 30) gives the period of incubation of the 
pigeon as> only 14 days, but it would seem as if he must here be 
referring to one of the smaller species, such as the Turtle Dove. 
Moore (1735, p. 11) in his famous “Columbarium” says: “The 
time of a pigeon s incubation, which trouble is equally divided between 
the cock and hen, except that the hen always sits all night, is nineteen 
or twenty days from the first egg, and seventeen or eighteen from the 
last,” with which statement, as regards the period of incubation, 
modern authorities are in accord. 
Our books at the present time contain several thousand records 
of the day of laying and the day of hatching of eggs, taken in the 
course of the regular daily routine. Since, however, the recording 
has not always been done at the same time of day, the period of 
accuracy may in reality sometimes be longer than a 24-hour period, as 
for example would be the case if the records were taken in the fore¬ 
noon of one day, and not again until the afternoon of the day follow¬ 
ing. We have, however, 118 records in which the time of hatching 
of both eggs is known within a 12-hour period; for those which hatched 
in the daytime the time of incubation of the egg is indeed often known 
