CLIMATIC CONDITIONS OF THE UNITED STATES. 367 



intensity is to be measured. The white surface absorbs but Httle 

 radiant energy, while the black one absorbs a large proportion of the 

 sunlight that reaches it. Both instruments are similarly affected by 

 alterations in the evaporating power of the air, due to whatever cause, 

 and the difference between their losses for any given time period is the 

 amount of water vaporized on account of the energy absorbed by the 

 black surface, over and above what is absorbed by the white one. 

 This difference is thus an approximate measure of the radiation inten- 

 sity for the given period, as this might accelerate evaporation from 

 moist exposed surfaces of the kind here employed. The instrument is 

 calculated to integrate the effects of sunshine throughout the time 

 period that occurs between the readings, and it is exceedingly sensitive 

 to relatively weak light intensities, so that it can give a wide range of 

 rates. The period of exposure may of course be made of any conven- 

 ient length. Attention should be called to the fact that the readings 

 are to be interpreted as the time-rates of work done in vaporizing water 

 from the standard surface, so that it thus becomes possible to consider 

 sunshine, from the cHmatological point of view, as to its power to do the 

 work of accelerating evaporation from the instrument. For a simple 

 term to denote this power we may use ''the evaporating pawer of the 

 sunshine." No doubt this expression can be shown to be faulty in 

 certain respects, if interest seems to lie in such a direction, but until 

 the very important cHmatological factor of light intensity begins to 

 attract serious attention it makes little difference in what terms we 

 emphasize its present neglect and its great importance. 



The sunshine records of the United States Weather Bureau consist 

 of observations on the number of hours of sunshine occurring each day 

 at each of the stations provided with the Marvin sunshine recorder. 

 This instrument is virtually a differential thermometer, having two 

 bulbs, the surface of one being a very good reflector and that of the 

 other being blackened. The automatic recording device records the 

 time periods when the blackened bulb has a temperature higher than 

 that of the other bulb by more than a certain small amount. During 

 periods of sunshine these two temperatures differ in this way. It is 

 thus seen that the instrument is not calculated to give any information 

 regarding comparative intensities of the impinging or absorbed radiant 

 energy. It simply records for each day the amount of time when the 

 sunshine was intense enough to produce the stated difference between 

 the temperatures of the two bulbs. While this recorder leaves much to 

 be desired, its records are probably more valuable than are periodic 

 ocular observations of the amount of cloudiness during daylight hours.^ 



1 In this connection see F. T. McLean. A preliminarj' study of climatic conditions in Maryland 

 as related to the growth of soy-bean seedlings, Physiol. Res. 2: 129-208, 1917. See also: 

 F. Merrill Hildebrandt, A method for approximating sunshine intensity from ocular observa- 

 tions of cloudiness, Johns Hopkins Univ. Circ, March, 1917, pp. 205-208.— Idem, 1921. 



