46 



Actual Humidity in Incubators. 



Having- thus established our manipulation of the absolute method, we 

 could with confidence use it in determining- the relative humidity in incu- 

 bators and in nests. A series of machines and several hens were set in 

 August for this special study. For the machines three facts were recorded 

 at each determination : (i) the humidity of the room and the correspond- 

 ing vapor pressure ; (2) the apparent humidity in the incubator by the wet 

 and dry bulb without fanning and the corresponding vapor pressure, two 

 or three readings for one determination ; (3) the actual humidity in the 

 incubator by the absolute method and the corresponding vapor pressure, 

 two or three readings for one determination. 



Perhaps some explanation should be made of the term "vapor pres- 

 sure." Every gas or vapor has an expansive power, a fact which may 

 be shown as follows : Tie tightly a thin rubber over the mouth of a glass 

 beaker, place it in the receiver of an air-pump, and exhaust the air from 

 the receiver. The rubber will be seen to bulge outward as the air from 

 around it is pumped away. Hence the air within the beaker has an expan- 

 sive power. This causes it to exert a pressure outward on the rubber. The 

 outside air had the same power in equal measure and as long as it was 

 present to exert its pressure on the top, the rubber being equally pressed 

 in both directions, was neither bulged outward nor depressed inward. 

 But as soon as the outside air was partly removed and its pressure re- 

 duced, the expansive power of the air within manifested itself. Now, air 

 possesses this property when not confined in a vessel, but expansion is 

 prevented by the weight of the air above. That water vapor has an 

 expansive power and exerts a pressure may be shown in a similar way. 

 The more vapor in the air at any given temperature the greater the pres- 

 sure it (the vapor) exerts. When vapor issues from the tea-kettle its 

 pressure is higher than that in the air around and hence that vapor ex- 

 pands and keeps on expanding till the vapor pressure throughout the room 

 is uniform. This equalization would occur even if the air were perfectly 

 motionless. It is much hastened by air currents. There are various ways 

 of determining the pressure of the vapor in the air at any time, but they 

 are all too involved to be given here. Suffice it to say that when the tem- 

 perature of the air and the weight of vapor in a cubic foot are known, 

 then by applying certain physical laws, and performing a long mathe- 

 matical calculation, we are able to determine the corresponding vapor 

 pressure. In this calculation correction is made for the contraction of the 

 air when entering the cold bottle A (Fig. 1). When this vapor pressure 

 is known we are able to state the natural tendency of the moisture. If 

 the vapor pressure outside the machine is greater than inside, then the 

 room moisture would by its greater pressure pass through the cracks into 

 the incubator. If on the other hand the pressure within is greater, then 

 the moisture within will pass outward. 



Five machines were examined in this test. The result is given in 

 Table No. XVII. : 



