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.