83 



more rapid the evaporation, the higher the humidity the slower the evap- 

 oration. Hence the cooling- produced on the wet thermometer is an 

 inverse measure of the humidity. 



As soon as these facts were correlated in this manner a secondary 

 but simple method of determining- humidity was at hand. A large number 

 of determinations by some absolute method was made, and the results 

 tabulated, and at the same time wet- and dry-bulb readings were taken 

 and set down in the same tables opposite the corresponding humidities. 

 When sufficient readings had been taken a law was established by which 

 the humidities and wet- and dry-bulb readings for intermediate tempera- 

 tures could be interpolated and the tables completed. When this had been 

 done humidity determinations became easy : it was only necessary to take 

 the wet- and dry-bulb readings and then refer to the tables for the humid- 

 ity, which had previously been determined. 



Fig. 2 — Hygrometer used in incubators during 1906. In 1907 two holes 

 were bored through the door frame of each incubator and long wet and dry 

 bulbs inserted. This was found to be more satisfactory as it left the egg tray 

 free, and the condition of the thermometer was always known, no trouble with 

 tilling the bottle and easier to read. 



In order that the wet bulb may be continually kept moist, it is pro- 

 vided with a close-fitting linen sack to which is attached some candle- 

 wick which dips into a small cup or bottle of water — the water travels 

 up the wick to the sack as the oil ascends a lampwick. 



Perhaps it should be mentioned that in making humidity tables as 

 above described the wet bulb was gently fanned to dissipate the vapor 

 being given off by it ; for if this were not done, and the air were very- 

 stagnant, that lying close around the wet bulb would become highly vapor- 

 charged, and the humidity determined would really be representative of 

 only that small amount of highly charged air, not of the air generally. 



4 BULL. 163 



