There are no pipes or other sources of heat in the egg chambers. All temperature regulation is by means of air heated (or cooled as the case may be) outside of the egg rooms and forced into the egg rooms by a motor driven cone fan, maintaining a steady current of air, the rate of movement of which may be varied at will. The air movement maintained will always be sufficiently brisk, however, to prevent an unevenness of temperature in different parts of the room. So simple is this that the reader will doubtless wonder why it was not developed earlier. The reason is that air subject to the climatic influences will, with any forced draft sufficient to equalize temperature, result in a fatal rate of evaporation. Sprinkling the air has not generally been thought practical because of the notion that air must not be used in the egg chamber but once, which involved quite a waste of heat necessary in warming a large bulk of air and evaporating sufficient water. Moreover, no means has, in the past, been available for making a sufficiently accurate measurement of the evaporating power of the air. The hair hygrometers commonly sold to incubator operators are known by scientists to be absolutely unreliable. The range between the wet and dry bulb thermometers was found in the Ontario experiments to give readings with and without fanning that varied 15 to 20 per cent. in relative humidity which, at the temperature of an egg chamber, would amount to a variation of three to four hundred of vapor pressure units, which, with the forced draught plan, would ruin a hatch of eggs in a few hours. The sling psychrometer as used by the U.S. Weather Bureau should, in the hands of an expert, give results making possible measurements accurate to two or three per cent. of relative humidity or forty to sixty units of vapor pressure. In contrast with these blundering instruments we now have available an instrument with which the writer has frequently determined vapor pressure accurately to within a range of two or three vapor pressure units and the instrument is capable of being constructed for even finer work. As it is only by means of air with the moisture content absolutely controlled that the use of a large room becomes possible, we can now see why this type of hatching remained so long undeveloped. By means of such vapor pressure control the large egg chamber is not only feasible but the rate of evaporation at once becomes subject to the control of the operator and we achieve a perfection in artificial incubation hitherto unattained.