"That's easy," said Edith. "The infertile eggs, when held before the small hole when the lamp is lighted inside the box, will look perfectly clear, same as a fresh one, while the fertile ones will show a small dark spot, which is known as the embryo. Of course, you have to learn to tell whether the embryo is living or dead, but that's easy to learn." "I think I could take care of an incubator all right," she continued. "The first thing you do is to see if it is running steadily at the desired temperature before filling it with eggs; then you must fill the whole tray at one time and not add fresh eggs to a tray after it's once started. The eggs must be turned twice daily after the second and until the nineteenth day. The eggs must also be cooled once daily from the seventh to the nineteenth day, depending on the weather." "Do you fix the lamps first, Edith, or turn the eggs?" asked Bob. "Oh, you must turn the eggs before you fix the lamps," she replied, "and, of course, the machine must be cared for at regular hours, just the same as your dairy cows, and the lamp and the wick must be kept clean at all times--otherwise you would not get a uniform heat." "When do you test the eggs?" asked Bob. "On the seventh and fourteenth days; after the eighteenth day you must not open the machine until the chicks are hatched." "If you'll look after the incubator for us, Edith, it'll save me a lot of time--particularly now when we want to start work on the new cow barn." "Will you let me run it all myself, Bob?" she asked, her eyes sparkling in anticipation. "I don't see why you can't do it all yourself. You understand it just as well as I do; besides, I've had no actual experience myself." They carefully filled the incubator with the eggs, making a record in a special book of the different breeds and the different breeders. "How are you going to mark them, Bob, to tell them apart?" asked Edith. "Oh, that's easy," said Bob. "You punch small holes between their toes and make a code of the marks, so you can tell which is which. "You can make ever so many combinations." "Doesn't that hurt them?" asked Edith. "No, not if it's done when they are very young--though the hole is a very small one, it never closes up, and you can always tell, by referring to your code, the age and breed of each chick. Later, of course, when they grow up, we'll put numbered aluminum bands on their legs, but when they're small the holes are better. "Just think, Bob, five hundred little chicks for me to look after. Won't it be perfectly splendid?" "You won't get five hundred, Edith. If we get sixty to seventy per cent, hatched, it will be as much as we can expect. Unless, of course, we have especially good luck and you might get as high as eighty or ninety per cent." "What will we do with the eggs that are not fertile?" she asked. "Oh, we'll boil those and feed them to the young chicks after they're hatched; they make good chicken feed."