Fahrenheit. Where the eggs are not kept longer than one week, it is not
necessary to turn them, especially if they are kept on end. If kept
longer than this it is safer to turn them once a day or once in two
days, handling them carefully so as not to crack any or to injure their
hatching qualities.
_Selecting the Eggs for Hatching._ Medium sized eggs are preferred for
this purpose. Therefore, the extremely large eggs and the very small
ones are thrown out. Rough shelled eggs or eggs with crooked or deformed
shells are likewise thrown out since they are not likely to hatch well.
Eggs that are badly soiled so that they cannot be tested easily are
washed but the clean eggs are not. All the eggs intended for incubation
purposes are sounded by striking them gently against one another in
order to detect and remove the cracked eggs. No selection is made on the
basis of color. The eggs may be white, creamy white or a blue, or bluish
green in color. At the present time a considerably less proportion of
the eggs show a blue tint than formerly. As the egg laying season
advances the eggs laid by the ducks tend to get a little larger.
_Temperature._ Up to the time of testing, that is, about the fifth day,
the incubator is run at a temperature of from 101 to 102 degrees. After
the fifth day the temperature is kept as near 103 as possible. The most
sensitive period for a duck egg is during the first 3 or 4 days of
incubation. If they are allowed to get too warm during this time the
germ may be killed while if the temperature is too low, development will
be retarded.
_Position of the Thermometer._ In figuring on the proper temperature at
which to run the incubator, the thermometer should be so placed that the
bulb is on a level with the top of the eggs, preferably touching a
fertile egg. If the thermometer bulb rests on an infertile egg the
temperature recorded will be lower than the actual temperature of
fertile eggs in the later stages of incubation, due to the animal heat
of the developing embryos, with the result that the machine would be
operated at too high a temperature.
_Testing._ It is common practice to make only one complete test. This is
done on the evening of the fifth day. Testing may be done by means of an
ordinary candling device such as is used with hens' eggs, each egg being
examined separately. To save time a piece of apparatus may be used which
is simple in construction and which simplifies the process of candling
considerably. This may be termed a