INCUBATION 25 
one. or two rows at the marked end of the 
trays and rolling the rest toward that end. 
By following this system of turning, at the 
end of two or three days the eggs will have 
passed directly across the tray and will have 
occupied every possible variation of tempera- 
ture which may be present in the machine. 
After the first twelve hours the eggs should 
be turned three times daily—6 a.m., 12 noon, 
and at 6 p.m.—until a chick is found to have 
pipped the shell; then the turning should 
cease. 
Ventilation. In the average house cellar 
where the ventilation is good, it will not be 
necessary to take the eggs out of the ma- 
chine and allow them to set for any length 
of time merely to cool them. The ventila- 
tion of a cellar may be judged by the presence 
or absence of odors. In the older types of 
incubators, where the ventilation problem 
had not been seriously considered, or in 
some cellars where the ventilation is insuffi- 
cient, it may be necessary to cool the eggs; 
but under most conditions it will not be 
necessary. The eggs, of course, will have to 
be taken out of the machine to turn them, 
