The discovery and locations of the minute organisms may be interesting to the scientist, but not at all necessary to the operator, who simply wants to be assured of the life and health of the germ. This he can readily determine from the increased size and gradual development of the circle; it, and the contents of the egg, now assume a darker shade. Up to this time I use no moisture, and the contents of the eggs have gradually evaporated and the air-cell proportionately enlarged. This air-cell is slightly enlarged till the tenth day, when no further evaporation should take place. About three days before hatching the rapidly developing duck will gradually diminish the size of the air-cell, leaving himself just room enough to work out. Nature, in the case of the old hen, provides for her own contingencies, while we must resort to art to obtain the same conditions. While incubating under the hen during the first few days, the egg evaporates rapidly. Then the pores gradually become coated with an oily secretion from the feathers of the hen until evaporation ceases. Now, we cannot successfully fill the pores of the eggs, it is too delicate an operation to attempt; but we can easily obtain the same conditions in another way, and that is to prevent the further evaporation of the egg by vaporizing water in the egg-chamber, so that evaporation will not take place. Exactly when this should be done is already known, but exactly how much is quite another thing, and depends largely upon the conditions of the atmosphere outside. The point is this: the humidity inside the egg-chamber must be the same, whatever the conditions are outside. If your machine is in a warm, dry room, heated by a fire, far more evaporating surface will be required than in a cool, dry cellar, for the reason that water vaporizes just in proportion to its heat; and as the circulating pipes upon which the water-pans rest must necessarily be much warmer in a cold room than in a warm one, of course more surface must be exposed in a warm than in a cold one. The operator will always have to use his judgment more or less in that. It may perplex the novice somewhat, but it is easily understood when one becomes accustomed to it. As a rule, in our machines, we introduce one moisture pan about the 18th day for both duck's and hen's eggs. It makes some difference whether a machine is run in a humid atmosphere near the seashore or in a dry, rarified atmosphere at an altitude in the country. Figure 5 represents the egg at the end of the fifth day, the circle enlarged, shaded darker in color; the whole egg being slightly darker in hue. [Illustration: FIGURE 5.--EGG AT END OF 120 HOURS.]