45 



hygrometers when placed in wet incubators was much less than when 

 under the hens and quite variable in any individual incubator. In the very 

 dryest of the dry the difference was almost the same as under the hens. 

 That variation in circulation affects the differential reading may be seen 

 from Table XVI. When testing the "egg" against the "frame" in the 

 room a little Ajax motor-fan was used to produce movement of air past 

 them. The distance of the fan from the hygrometers was varied, hence 

 the rate of, circulation was likewise varied. It will be seen that for both 

 low and high humidities (temperature 96 in both cases) there is a rate of 

 circulation which gives a maximum differential reading, and also that 

 either faster or slower circulation will reduce the difference. Therefore 

 since the differential was lower in the wet incubators than under the hens 

 the circulation was different, either faster or slower. Likewise since the 

 differential in the dry machines was equal to that under the hens, the cir- 

 culation must have been different, for the same amount of circulation pro- 

 duces a much greater differential in a dry than in a moist atmosphere. 

 Whether the circulation in the incubators is greater or less than that 

 under hens we are not able to say from direct measurement, but we have, 

 however, indirect proof that seems to indicate unmistakably that the incu- 

 bator circulation is considerably the slower, proof that came to us during 

 our study of nest humiditv by the absolute method. 



Humidity by the Absolute Method. 



To determine humidity absolutely has given us more trouble than 

 any other part of the work. In 1906, being busy with other problems, and 

 having only a short time to devote to it, it baffled us entirely. Although 

 it looks easy in description, as given on pages 29 to 32, it is difficult 

 in application. When beginning to use it last year we made a number of 

 determinations of the room humidity by it one after another, to test the 

 method. The variations were so great that it was evident that something 

 was wrong in the manipulation. The drying tubes were weighed, 500 

 cc. of air drawn through them, and then re-weighed. The operation was 

 repeated over and over several times in succession, but often the tubes 

 would gain two or three times as much in weight as the time before, 

 while the wet- and dry-bulb readings would show constant humidity. In 

 the spring of 1907, with more time at our disposal, it was discovered that 

 the variations were largely due to condensation on the tubes of moisture 

 from the breath when it happened to be directed against the cold glass. 

 From that time on a mask was worn by the operator so that the breath 

 could not possibly strike the tubes. As a further precaution rubber gloves 

 were worn, so that no perspiration from the hands could condense on the 

 tube. With these precautions we were able to determine the humidity 

 of the room correctly by this method. Seventeen comparisons extending 

 from May to August srave the following results : 



Wet and dry bulb, average relative humidity, 49.5 per cent. 



Absolute method, average relative humidity, 49.3 per cent. 



