30 BURTON EDWARD LIVINGSTON 



seriously affect the readings of a rain gauge. To illustrate, a 

 small pan of water loses relatively (per unit of surface) more by 

 evaporation than does a large pan, while the size of rain gauge 

 funnels is without appreciable effect upon the amount of water 

 taken in, per unit of funnel area. It may be remarked, also, 

 that the very presence and operation of an evaporating surface 

 decreases the evaporating power of the air in its vicinity, while 

 the receiving or absorbing action of the rain funnel is quite 

 without influence towards decreasing rainfall. This is an im- 

 portant consideration for those who regard units of depth of 

 evaporation and precipitation as comparable units. The prop- 

 osition is much more complicated than here indicated, but 

 limited space precludes a more thorough analysis, 



Since it is aimed simply to measure the amount of water lost 

 from a specified standard evaporating surface, it is clear that 

 the only rational units thus to be employed are those of weight, 

 or mass. Volume units are as satisfactory as those of weight 

 only when the relatively small variation in specific gravity 

 caused by changing temperature may be neglected; which may 

 be safely done in practically all cases where atmometric measure- 

 ments are employed. The investigator should be conscious of 

 such neglect, however. Units of depth cannot be conveniently 

 employed excepting with free water surfaces; other forms of 

 atmometer lend themselves more readily to the employment of 

 weight or volume units. In the case of pans or tanks of water, 

 depth units (which are to be avoided, in any event, because of 

 the widespread but mistaken idea that these are directly com- 

 parable to similar units of precipitation) may of course be trans- 

 formed into weight or volume units when the necessary co- 

 efficient is given — but always for some specific size, shape, etc., 

 of pan. 



Of course the most essential principle in this regard is that 

 the same units must be used for all the instruments the readings 

 of which are to be compared. 



(To be continued.) 



