42 



THE HABITAT 



structure of the instrument. Continued psychrometric tests- demonstrate 

 that the marg-in of error is well within the efficient difference for humidity, 

 which is taken to be 5 per cent. In the field tests of the past summer, two 

 psychrographs placed side by side in the same habitat did not vary i per 

 cent from each other. The same instruments when in different habitats 

 did not deviate more than i per cent from the psychrometric values, ex- 

 cept when the air approached saturation. For humidities above 90 per cent, 

 the deviation is considerable, but as these are temporary and incident upon 

 rainfall, the error is not serious. For humidities varying from 10-85 P^^ 

 cent, the psychrograph is practically as accurate as the psychrometer. Per 

 cents below 10 are rare, and no tests have been made for them. 



Fig. 8. Instrument shelter, showing thermograph and psychrograph 

 in position. 



67. Placing the instrument. The psychrograph should be located in a 

 place where the circulation of the air is typical of the station observed. A 

 satisfactory shelter will screen the instrument from sun and rain, and at 

 the same time permit the air to pass freely through the perforations of 

 the metal case. The form shown in figure 8 meets both of these condi- 

 tions. A desirable modification is effected by fastening a strip about the 

 cover of such depth as to prevent the sun's rays from striking the case ex- 

 cept when the sun is near the horizon. A cross block is fastened on the 



