INCUBATION 
The Box Type of Incubator In Actual Use. 
Although the fact is not so advertised and frequently not 
recognized even by the makers, the success of existing incu- 
bators is directly proportional to the extent with which they 
control evaporation. In order to show this I have only to call 
attention briefly to two or three of the most successful types 
of incubators on the market. 
Let me first repeat that evaporation increases with increas- 
ed air currents and with decreased vapor pressure. Now, 
the vapor pressure undergoes all manner of changes with the 
passing of storm centers and the changes of prevailing winds. 
But there is a general tendency for vapor pressure to increase 
with increase in outside temperature. Now, the movement of 
air in all common incubators depends upon the draft principle 
and the greater the difference in machine temperature and 
outside temperature the greater will be this draft. Thus, we 
have two factors combining to cause variation in the rate of 
evaporation. The tendency for the rate of airflow to vary is 
diminished when a cellar is used for an incubator room, but 
the cellar does not materially remedy the climatic variation 
in vapor pressure. 
The general tendency of incubators as ordinarily construct- 
ed, is to dry out the eggs too rapidly. With a view of counter- 
acting this, water is placed in pans in the egg room. A sur- 
face of water exposed to quiet air does not evaporate as 
fast as one might think, as is easily shown by the fact that 
air above rivers, lakes and even seas is frequently far from 
the saturation point. The result of the moisture pan with a 
given current of air is that the vapor pressure is increased 
a definite amount, but by no means is it regulated or made 
uniform, Inasmuch as too much shrinking is the most preva- 
lent fault in box incubators, the use of moisture is on the 
whole beneficial, but in hot, murky weather, with less circula- 
tion and higher outside vapor pressure, the moisture is over- 
done and the operator condemns the system. 
The subject not being clearly understood and no means be- 
ing available for vapor pressure determinations, the system 
results in confusion and disputes. When the felt diaphragm 
machine was brought into the market it was advertised as a 
no-moisture machine. The result of the diaphragm is that of 
choking off air movement and consequently reducing evapora- 
tion. This gives exactly the same results as the use of mois- 
ture, but the machine is easier to operate and seemed to do 
away with the vexatious moisture problem which, together 
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