By a further study from the Ontario data of the relation of the evaporation to the results in livable chicks, it can be readily observed that all good hatches have evaporation centering around the 12 per cent. moisture loss, and that all lots with evaporations above 15 per cent. hatch out extremely poor.
The general averages of the machines supplied with some form of moisture was 35 per cent. of all eggs set, in chicks alive at four weeks of age, while the machines ran dry gave only 20 per cent. of live chicks at a similar period.
Now, I wish to call attention to a further point in connection with evaporation. If the final measure of the loss of weight by evaporation were the only criterion of correct conditions of moisture in the chick's body, the hatches that show 12 per cent., or whatever the correct amount of evaporation may be, should be decidedly superior to those on either side. That they are better, has already been shown. But they are far from what they should be. An explanation is not hard to find. The correct content of moisture is not the only essential to the chick's well being at the moments of hatching, but during the whole period of incubation. Under our present system of incubation, the chick is immediately subject to the changing evaporation of American weather conditions. The data for that fact, picked at random, will be of interest. The following table gives the vapor pressure at Buffalo, N. Y., for twenty consecutive days in April:
| April 1 | 170 |
| April 2 | 130 |
| April 3 | 95 |
| April 4 | 103 |
| April 5 | 110 |
| April 6 | 106 |
| April 7 | 154 |
| April 8 | 183 |
| April 9 | 245 |
| April 10 | 311 |
| April 11 | 342 |
| April 12 | 286 |
| April 13 | 219 |
| April 14 | 248 |
| April 15 | 217 |
| April 16 | 193 |
| April 17 | 241 |
| April 18 | 306 |
| April 19 | 261 |
| April 20 | 204 |
Supposing a hatch to be started at the beginning of the above period, by the end of the first week, with the excessive evaporation, due to a low vapor pressure, the eggs would all be several per cent. below the normal water content; the fact that the next week was warm and rainy, and the vapor pressure rose until the loss was entirely counterbalanced, would not repair the injury, even though the eggs showed at the end of incubation exactly the correct amount of shrinkage. A man might thirst in the desert for a week, then, coming to a hole of water fall in and drown, but we would hardly accept the report of a normal water content found at the post-mortem examination as evidence that his death was not connected with the moisture problem.
The change of evaporation, due to weather conditions, is, under hens, less marked than in incubators. This is because there are no drafts under the hen, and because the hen's moist body and the moist earth, if she sets on the ground, are separate sources of moisture which the changing humidity of the atmosphere does not affect. Among about forty hens set at different times at the Utah Station and the loss of moisture of which was determined at three equal periods of six days each, the greatest irregularity I found was as follows: 1st period, 5.81 per cent; 2d period, 3.86 per cent; 3d period, 6.15 per cent. Compare this with a similar incubator record at the same station in which the loss for the three periods was 5.63, 9.18 and 2.15.
I think the reader is now in position to appreciate the almost unsurmountable difficulties in the proper control of evaporation with the common small incubator in our climate. It is little wonder that one of our best incubator manufacturers, after studying the proposition for some time, threw over the whole moisture proposition, and put out a machine in which drafts of air were slowed down by felt diaphragms and the use of moisture was strictly forbidden.
The moisture problem to the small incubator operator presents itself as follows: If left to the mercies of chance and the weather, the too great or too little evaporation from his eggs will yield hatches that will prove unprofitable. In order to regulate this evaporation, he must know and be able to control both vapor pressure and the currents of air that strike the eggs. Now he does not know the amount of vapor pressure and has no way of finding it out. The so-called humidity gauges on the market are practically worthless, and even were the readings on relative humidity accurately determined, they would be wholly confusing, for their effect of the same relative humidity on the evaporation will vary widely with variations of the out-of-door temperature.
If the operator knows or guesses that the humidity is too low, he can increase it by adding water to the room, or the egg chamber, but he cannot tell when he has too much, nor can he reduce the vapor pressure of the air on rainy days when nature gives him too much water. As to air currents he is little better off—he has no way to tell accurately as to the behavior of air in the egg chamber and changes in temperature of the heater or if the outside air will throw these currents all off, since they depend upon the draft principle.
Taking it all in all, the man with the small incubator had better follow the manufacturer's directions and trust to luck.