208 POULTRY BREEDING 
warm weather the first week of incubation, 102° the sec- 
ond and 103° the third week. With the hang-up ther- 
mometer the readings will run from 103 to 108°, depend- 
ing on the machine and the season of the year, to obtain 
the same temperature, the object being the same degree 
of heat as in natural incubation. The heat should be ap- 
plied to the top of the eggs from above. Bottom or side 
heat will not do the work. No incubator using other 
than top heat ever made a commercial success. The 
maximum quantity of heat that can be apphed with safety 
is 108° and this not for very long at a time. High tem- 
peratures cause too rapid growth and low temperatures 
retard growth. In either case when extremes of tem- 
perature are prolonged the chick dies in the shell. The 
heat used must be distributed so the eggs will develop 
equally and will all hatch at once. With bad distribu- 
tion, hot and cold spots in the egg-chamber, the eggs are 
greatly weakened and, where variation is too wide, are 
killed outright. 
“The measured quantity of air required to incubate has 
as yet never been determined, although many workers are 
engaged on the problem. Eggs placed in tightly-corked 
bottles in an incubator live but a short time, even in the 
beginning of incubation. The embryo breathes through 
the shell until the time of exclusion. Experience has 
taught ‘the great necessity of ventilation in securing 
healthy, hardy chicks. Not only must the incubator be 
ventilated, but the ventilating current must not be con- 
taminated by leaky heaters or bad air in the room where 
the incubator stands. The embryos are particularly sen- 
sitive to gaseous or vapor influences. Prof. Elford at 
McDonald College found that the vapor of kerosene oil 
from an ordinary tank in the hatching room reduced the 
