This is about the whole story save for results. The incubator men pay back three chicks for four eggs, and take their profits by selling the extra chicks that are hatched above the 75 per cent. This statement is in itself so astonishing and yet convincing, that to add that the hatch runs between 85 per cent. and 90 per cent. of all eggs set, and that the incubators of the Nile Delta hatch about 75,000,000 chicks a year seems almost superfluous. As for the explanation of the results of the Egyptian incubators compared with the American kerosene lamp type, I think it can best be brought about by a consideration in detail of the scientific principles of incubators.
Principles of Incubation.
HEAT.—To keep animal life, once started, alive and growing, we need: First, a suitable surrounding temperature. Second, a fairly constant proportion of water in the body substance. Third, oxygen. Fourth, food.
Now, a fertile egg is a living young animal and as such its wants should be considered. We may at once dispose of the food problem of the unhatched chick, by saying that the food is the contents of the egg at the time of laying, and as far as incubation is concerned, is beyond our control.
In consideration of external temperature in its relation to life, we should note: (A) the optimum temperature; (B) the range of temperature consistent with general good health; (C) the range at which death occurs. Just to show the principle at stake, and without looking up authorities, I will state these temperatures for a number of animals. Of course you can dispute the accuracy of these figures, but they will serve to illustrate our purpose:
| External Optimum Point | External Healthful Range | External Fatal Range | Internal Optimum Point | Internal Fatal Range | |
| Man | 70 | 0 to 100 | 50 to 140 | 98 | 90 to 106 |
| Dog | 60 | 70 to 140 | 70 to 140 | 101 | 95 to 110 |
| Monkey | 90 | 30 to 140 | 30 to 140 | 101 | 95 to 108 |
| Horse | 80 | 20 to 120 | 20 to 120 | 99 | 95 to 105 |
| Fowl | 80 | 20 to 140 | 20 to 140 | 107 | 100 to 115 |
| Newly hatched chick | 90 | 70 to 100 | 40 to 120 | 108 | 100 to 115 |
| Fertile egg at start of incubation | 103 | 32 to 110 | 31 to 125 | 103 | 31 to 125 |
| Egg incubated three days | 103 | 98 to 105 | 80 to 118 | 103 | 95 to 118 |
| Egg incubated eighteen days | 103 | 75 to 105 | 50 to 118 | 106 | 98 to 116 |
This table shows, among other things, that we are considering in the chick not a new proposition to which the laws of general animal life do not apply, but merely a young animal during the process of growth to a point where its internal mechanism for heat control, has power to maintain the body temperature through a greater range of external temperature change.
In the cooling process that occurs after laying the living cells of the egg become dormant, and like a hibernating animal, the actual internal temperature can be subjected to a much greater range than when the animal is active. After incubation begins and cell activity returns, and especially after blood forms and circulation commences, the temperature of the chick becomes subject to about the same internal range as with other warm blooded animals.
In the case of fully formed animals, the internal temperature is regulated by a double process. If the external temperature be lowered, more food substance is combined with oxygen to keep up the warmth of the body, while, if the external temperature be raised, the body temperature is kept low by the cooling effects of evaporation. This occurs in mammals chiefly by sweating. Birds do not sweat, but the same effect is brought about by increased breathing. Now, the chick gradually develops the heat producing function during incubation, until towards the close of the period it can take care of itself fairly well in case of lowered external temperature. The power to cool the body by breathing is not, however, granted to the unhatched chick, and for this reason the incubating egg cannot stand excess of heat as well as lack of it.
The practical points to be remembered from the above are: