Fourth: It should be remembered, however, that eggs are very poor conductors of heat, and if the temperature change is not great several hours of exposure are required to bring the egg to the new temperature. Temperature is the most readily observed feature about natural incubation and its control was consequently the first and chief effort of the early incubator inventors. A great deal of experimental work has been done to determine the degree of temperature for eggs during incubation. The temperature of the hen's blood is about 105 to 107 degrees F. The eggs are not warmed quite to this temperature, the amount by which they fail to reach the temperature of the hen's body depending, of course, upon the surrounding temperature. 103 degrees F. is the temperature that has been generally agreed upon by incubator manufacturers. Some of these advise running 102 degrees the first week, 103 degrees the second, 104 degrees the third. As a matter of fact it is very difficult to determine the actual temperature of the egg in the box incubator. This is because the source of heat is above the eggs and the air temperature changes rapidly as the thermometer is raised or lowered through the egg chamber. The advice to place the bulb of the thermometer against the live egg is very good, but in practice quite variable results will be found on different eggs and different parts of the machine. With incubators of the same make, and in all appearances identical, quite marked variation in hatching capacity has been observed in individual machines. Careful experimentation will usually show this to be a matter of the way the thermometer is hung in relation to the heating surfaces and to the eggs. Ovi-thermometers, which consists of a thermometer enclosed in the celluloid imitation of an egg, are now in the market and are perhaps as safe as anything that can be used. As was indicated in the previous section greater care in temperature of the egg is necessary in the first half of the hatch. The temperature of 102 degrees F. as above given is, in the writer's opinion, too low for this portion of the hatch. An actual temperature of 104 degrees at the top of the eggs will, as has been shown by careful experimental work, give better hatches than the lower temperature. Moisture and Evaporation. The subject of the water content of the egg and its relation to life, is the least understood of poultry problems.