These figures are based on incubator data. Eggs set under hens usually give a hatchability of 50 per cent. to 65 per cent., and livability of 70 per cent. to 80 per cent. The reason for the greater livability is that the real hatchability of the eggs is 70 per cent. to 75 per cent., and is reduced by mechanical breakage. The hatchability of eggs varies with the season. This variation is commonly ascribed to nature, it being stated that springtime is the natural breeding season, and therefore eggs are of greater fertility. While there may be a little foundation for this idea, the chief cause is to be found in the manner of artificial incubation, as will be discussed in a later section of this chapter. The following table is given as the seasonable hatchability for northern states. This is based on May hatch of 50 per cent: January 38 July 40 February 42 August 40 March 47 September 42 April 49 October 43 May 50 November 40 June 46 December 35 Most people have an exaggerated idea of the hen's success as a hatcher. I have a number of records of hen hatching with large numbers of eggs set, and they are all between 55 per cent. and 60 per cent. The reasons the hen does not hatch better are as follows: 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.