INCUBATION 
the thermometer is raised or lowered through the egg cham- 
ber. The advice to place the bulb of the thermometer against 
the live egg is very good, but in practice quite variable results 
will be found on different eggs and different parts of the 
machine. 
With incubators of the same make, and in all appearances 
tdentical, quite marked variation in hatching capacity has 
been observed in individual machines. Careful experimenta- 
tion will usually show this to be a matter of the way the ther- 
mometer is hung in relation to the heating surfaces and to 
the eggs. Ovi-thermometers, which consists of a thermometer 
enclosed in the celluloid imitation of an egg, are now in the 
market and are perhaps as safe as anything that can be used. 
As was indicated in the previous section greater care in 
temperature of the egg is necessary in the first half of the 
hatch. The temperature of 102 degrees F. as above given is, 
in the writer’s opinion, too row for this portion of the hatch. 
An actual temperature of 104 degrees at the top of the eggs 
will, as has been shown by careful experimental work, give 
better hatches than the lower temperature. 
Moisture and Evaporation. 
The subject of the water content of the egg and its relation 
to life, is the least understood of poultry problems. 
The whole study of the water content of the egg during in- 
cubation hangs on the amount of evaporation. Now, the rates 
of evaporation from any moist object is determined by two 
factors: vapor pressure and the rate of movement of the air 
past the object. ‘As incubation is always carried on at the 
same temperature, the evaporating power of the air is directly 
proportioned to the difference in the vapor pressure of water 
at that temperature, and the vapor pressure of the air as it 
enters the machine. Thus, in order to know the evaporative 
power of the air, we have only to determine the vapor pres- 
sure of the air and to remember that the rate of evaporation 
is in proportion to this pressure, i. e.: when the vapor pressure 
is high the evaporation will be slow and the eggs remain too 
wet, and when the vapor pressure is low the eggs will be ex- 
cessively dried out. 
The reader is probably more familiar with the term relative 
humidity than the term vapor pressure, but as the actual 
significance of relative humidity is changed at every change 
in outside temperature, the use of this term for expressing 
a evaporating power of the air has led to no end of con- 
‘usion. 
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