The following are averages of the monthly vapor pressures at four points in which we are interested: 

Buffalo,  St. Louis,  San Fran-  Cairo
Month      N.Y.          Mo.       cisco.    Egypt
January     87          98         311      279
February    81          94         310      288
March      138         224         337      287
April      171         283         332      311
May        301         423         317      328
June       466         550         345      365
July       546         599         374      413
August     496         627         382      435
September  429         506         389      372
October    285         327         342      365
November   271         225         285      321
December   143         133         243      397

A study of the extent of daily variations is also of interest.  As a general thing they are less extreme in localities when the seasonal variations are also less.  In Cairo, however, which has a seasonal variation greater than San Francisco, the daily variations during the hatching season are much less than in California.  This is due to a constant wind from sea to land, and an absolute absence of rainfall, conditions for which Egypt is noted.

Nearness to a coast does not mean uniform vapor pressure, for with wind alternating from sea to land, it means just the opposite.

As will be readily seen the months in spring which give the best hatches, occupy a medium place in the humidity scale.  The fact that both hens and machines succeed best in this period, is to me very suggestive of the possibility that with an incubator absolutely controlling evaporation, much of the seasonal variation in the hatchability would disappear.

The uniform humidity of the California coast is shown in the above table.  This is not inconsistent with the excellent results obtained at Petaluma.

The Egyptian hatcher in his long experience has learned just about how much airholes and smudge fire are necessary to get results.  With these kept constant and the atmosphere constant, we have more nearly perfect conditions of incubation than are to be found anywhere else in the world, and I do not except the natural methods.  The climatic conditions of Egypt cannot be equaled in any other climate, but as will be shown in the last section of this chapter, their effect can be duplicated readily enough by modern science and engineering.

Mr. Edward Brown, who was sent over here by the English Government to investigate our poultry industry, was greatly surprised at our poor results in artificial incubation.  Compared with our acknowledged records of less than 50 per cent. hatches, he quotes the results obtained in hatching 18,000 eggs at an English experiment station as 62 per cent.  I have not obtained any data of English humidity, but it is undoubtedly more uniform than the eastern United States.

Ventilation—­Carbon Dioxide.