Another essential condition for a successful hatch with the incubator is proper ventilation. Most machines are provided with something for the admission of fresh air. It is also necessary that the room containing the incubator be aired frequently to admit fresh air, but not in such a manner that the eggs will become chilled, or that too much air will be admitted so as to carry off the moisture. Most incubators carry proper directions so as to provide against this danger. The third condition necessary for proper running of the incubator is the maintenance of a constant temperature. It is usually advisable to have the temperature about 102 degrees F. the first week, 103 degrees the second week, and 104 degrees the third week. Any serious deviation from 103 degrees will result in a poor hatch. Now, then we will conclude this chapter with a few pointers on running the incubator during the hatching period. It is always well to run the incubator a day or two empty, so as to get the idea of how to best maintain the required temperature. Clean the lamp thoroughly before running, and then during the hatch, always tend to the lamp after the eggs have been turned, and never before, because if the hands are smeared with kerosene, some of this is bound to be left on the shells, which will result in a poor hatch. Do not allow the lamp to be in a draft, and clean the wick every day, so as to prevent the lamp from smoking. After the first few days, the eggs should be turned at least once a day, and even better, twice a day. This is to imitate the mother hen, who does this on the nest. If the eggs are not turned, then the embryo of the chick will adhere to the shell of the egg, and either die, or be crippled beyond hope. We must imitate the mother hen again in the artificial incubation of chicks, in regard to cooling. We saw in the previous chapter, how the hen leaves her nest occasionally to take food and water, with no ill effects on the hatch. Likewise we must cool the eggs in the incubator. This can usually best be accomplished while turning the eggs. Five or ten minutes a day are usually sufficient for the proper cooling, unless it is near the end of the hatch, when twenty or thirty minutes can be used for this purpose with safety.