First:  Actual infertile eggs—­usually, running about 10 per cent. in the best season of the year.

Second:  Mechanical breakage.

Third:  Eggs accidentally getting chilled by rolled to one side of the nests, or by the sick, lousy or crazy hens leaving the nests or standing up on the eggs.

Fourth:  Eggs getting damp from wet nests, dung or broken eggs; thus causing bacterial infection and decay.

The last three causes are not present in artificial incubation.  From my observation they cause a loss of 15 per cent. of the eggs that fail to hatch, when hens are managed in large numbers.  This would properly credit our hens with hatches running from 70 per cent. to 75 per cent., which, for reasons later explained, is not equal to hatches under the best known conditions of artificial incubation.

The assumption that the hen is a perfect hatcher, even barring accidents and the inherited imperfection of the egg, is not, I think, in harmony with our general conception of nature.  Not only are eggs under the hens subject to unfavorable weather conditions, but the hen, to satisfy her whims or hunger, frequently remains too long away from the eggs, allowing them to become chilled.

For directions of how to manage setting hens, consult the Chapter on “Poultry on the General Farm.”

The Wisdom of the Egyptians.

Up to the present there have been just three types of artificial incubation that have proven successful enough to warrant our attention.  These are: 

First, the modern wooden-box-kerosene-lamp incubator which is seen at its best development in the United States.

Second, the Egyptian incubator of ancient origin, which is a large clay oven holding thousands of eggs and warmed by smouldering fires of straw.

Third, the Chinese incubator, much on the principle of the Egyptian hatchery, but run in the room of an ordinary house, heated with charcoal braziers and used only for duck eggs.

I have no accurate information on the results of the Chinese method, and as it is not used for hen eggs, we will confine our attention to the first two processes only.

I do not care to go into detail in discussing makes of box incubators, but I will mention briefly the chief points in the development of our present machines.

The first difficulties were in getting lamps, regulators, etc., that would give a uniform temperature.  This now has been worked out to a point where, with any good incubator and an experienced operator, the temperature of the egg chamber is readily kept within the desired range.

These are two principal types of box incubators now in use.  In the earliest of these, the eggs were heated by radiation from a tank of hot water.  These machines depended for ventilation or, what is much more important, evaporation, upon chance air currents passing in and out of augur holes in the ends or bottom of the machine.