18 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



the ground. The standard type of shelter is double-roofed and has 

 a partly open floor, and walls made of slats which overlap to ex- 

 clude light from any overhead source without causing complete 

 stagnation of the air within. 



Modifications which would still more fully overcome the heating 

 of the shelter have been proposed by Koppen (23), who would pro- 

 vide artificial air circulation, but such provisions will hardly be 

 necessary for any ordinary thermometric work. On the other hand, 

 the observer can not be too strongly urged to provide shelters which 

 will give the maximum of light protection without preventing the 

 natural air currents from coming in contact with the thermometers. 

 To obtain true nocturnal temperatures near the ground it may be 

 desirable to use a shelter without a floor, so that radiation from 

 below the thermometers is retarded as little as possible. In the 

 work at the Fremont Experiment Station, Bates has found that a 

 shelter for ground temperatures need be no more than a hood, fully 

 open to the north and below the thermometers. If there should be 

 considerable reflection from the north at midday, it could be largely 

 eliminated by an absorbing screen set a foot or two from the hood. 



Standardizing Thermometers. 



The present possibilities of correlation between temperatures and 

 plant behavior do not justify the greatest precision in thermometry. 

 Units smaller than 1° may be ignored in field work, for all practical 

 purposes, though personal taste may dictate that tenths of degrees 

 be recorded. The essential thing is that only reliable thermometers 

 be used, as the errors in cheap thermometers are not uncommonly as 

 great as the difference between two conditions which are being 

 studied. Even the standard types of maximum, minimum, and mer- 

 curial thermometers may well be critically examined and compared 

 with a standard before being used. The Bureau of Standards (37) 

 calibrates such instruments at a nominal cost. 



With recording apparatus, such as the air thermograph, adjust- 

 ment takes the place of standardization. The use of any such re- 

 corder, without thermometers to check its accuracy at frequent in- 

 tervals, can not be recommended. 



Maximum, Minimum, and Current Temperatures. 



Where only maximum and minimum thermometers are available, 

 of the standard Weather Bureau pattern, the maximum and mini- 

 mum temperatures for the preceding 24 hours should be recorded 

 once each day, either before 10 a. m. or after 4p.m„ and at the same 

 time the current temperature, as indicated by the minimum ther- 

 mometer, should be recorded, also the time of the observation. The 

 current temperature is principally of value for making a thermo- 

 graph correction, and the height of the thermograph pen should 

 herefore be recorded at the same time. 



