558 THE MONTHLY BULLETIN. 



SELECTING A HYGROMETER. 



By E. Ralph de Ong, University Farm, Davis, Cal. 



The humidity of the air is being recognized more and more as of 

 great value in modern horticultural practices. Some fumigators are 

 placing the hj^grometer on the same basis as the thermometer in guiding 

 their work in citrus fumigation, and in frost fighting the value of a 

 definite knowledge of the dew point and the humidity of the air has 

 long been recognized — both of these factors being determined by the use 

 of the hygrometer. 



The term hygrometer is applied to all instruments used to determine 

 the amount of moisture in the air. The standard for these measure- 

 ments is the wet and dry bulb thermometer, or psychrometer, as it is 

 generally spoken of. This instrument is recommended by the Weather 

 Bureau for all accurate determinations of humidity. Two ther- 

 mometers are firmly fastened to a metal or wooden back, the bulb of 

 one thermometer being incased in muslin or fine silk, to be wet when 

 used ; the other bulb, as the name indicates, is to be dry. The instru- 

 ment is whirled rapidly in the air for forty or fifty seconds and the 

 difference between the readings of the two thermometers is noted ; from 

 this difference and the temperature of the air at the time the reading is 

 taken, the humidity can be determined by carefully worked out tables, 

 which may be obtained from the Weather Bureau or the houses carrying 

 scientific instruments. 



There are several forms of hygrometers on the market, the cheapest 

 being a dial instrument which sells for five or six dollars ; but this form 

 is not accurate enough for frost forecasting or for any work where an 

 error of a few points would be serious. One instrument, when com- 

 pared at different times in the same day, showed variations of 10 per 

 cent, 16 per cent and 21 per cent from that indicated by the wet and 

 dry bulb hygrometer. There are dial hygrometers on the market much 

 more accurate than the ones just referred to, but even these should be 

 corrected occasionally with a sling psychrometer. 



Another form of instrument which is cpiite popular is a stationary 

 hygrometer, having a dry bulb and a wet bulb thermometer, the l)ulb 

 of the latter being connected — generally by cotton wicking — with a 

 reservoir of water. No provision is made for ventilation and hence the 

 reading can not be as accurate as it would be with a forced circulation 

 of air, for with no air currents the atmosphere surrounding the wet 

 cloth is soon saturated with moisture, thus giving an incorrect reading 

 of the thermometer, as it would show more moisture present than was 

 actually the case in the surrounding atmosphere. Besides, the wicking 

 on the bulb is very apt to become clogged with mineral matter from the 

 water and from dust, so that water is not evaporated as freely, and 

 another element of error is added. One instrument of this type which 

 had been in operation for a year or more was compared with a sling 

 psychrometer in the following way : 



