36 



It will be observed that the humidity of the dry machines varies 

 within narrow limits, likewise that of the wet machines, but there is a 

 great difference between the former as a class and the latter, the relative 

 humidity of the latter being- about one-half higher than that of the former. 



Relative Humidity under Hens. 



We had not proceeded very far, however, when we became convinced 

 that if our work was to have its greatest value, we must learn the con- 

 ditions existing - in the hen's nest. To determine the humidity in the nest, 

 we devised the hygrometer shown in Fig. 4 (see page 39). It consists of 

 an eggf of brass strainer gauze held in shape by two perforated discs, and 

 fitted with two tin tubes through which the wet- and dry-bulb thermo- 

 meters could be inserted. To determine the humidity in the nest the 

 "egg" was to be inserted beneath the hen, the thermometers projecting 

 so that the readings could be taken. But I feared the vapor from the 

 wet bulb would saturate the air under the hen. To learn if this were 

 possible it was necessary to know the volume of air among the eggs in 

 the nest, the amount of vapor that air was capable of holding at ioo Q 

 (the temperature of the nest) and the amount of water on the sack of the 

 wet bulb. If the latter amount was equal to or greater than the former 

 then it would be possible, other conditions favoring, for the egg hygro- 

 meter to saturate the air in the nest. To g"ain some idea of the quantity of 

 air in the nest a circular, flat-bottomed dish, with upright sides, was pro- 

 cured which just held 13 eggs in one layer. Water was poured infill the 

 eggs were just covered. It took 42.5 cubic inches. This represents the air 

 space between the eggs. Then of course something - had to be allowed for 

 the extra air space caused by the presence of the hen's legs and breast 

 between the eggs. We thought that 17.5 cubic inches would be sufficient, 

 making a total of 42.5 plus 17.5, or 60 cubic inches. Turning up our 

 humidity tables we found that 1 cubic foot of air at ioo° was capable of 

 holding 19.8 grains, whence by calculation 60 cubic inches would hold 

 .68 grains, or almost exactly two-thirds of a grain. Then weighing the 

 thermometer before and after wetting, we found that the sack absorbed 

 1.27 grains, or nearly twice the saturation amount for the air in the nest. 

 Hence if the vapor from the wet bulb were not dissipated too rapidly it 

 should saturate the nest air. In proof of this argument the hygrometer 

 was placed in a rubber-stoppered bottle containing incidentally just half 

 as much air as the nest, the thermometers projecting through holes in the 

 stopper. In three hours' time the humidity had risen from 62. 9 to 95.2 per 

 cent., pretty close to saturation, and the sack was still thoroughly wet. 



Knowing thus the behavior of the hygrometer in a closed-up stagnant 

 air, we next placed it under a hen. Would it saturate the air there? 

 Here are the readings and remarks : 



