148 GEOLOGICAL SURVEY OF NEW JERSEY. 



eateles lake shows 2.3 inches less than the forested Hudson ; and 

 the Connecticut, with fifty-three per cent, of forests, shows 

 larger evaporation than the Merrimac, with but forty per cent.; 

 while the highest evaporation in the table is shown by the well 

 forested Batsto, Great Egg Harbor and Savannah catchments. 

 , Indeed, the variation in evaporation can only be satisfactorily 

 accounted for by the differences of rain-fall and temperature. 

 We have been able to determine accurately the effect of varying 

 rain-fall from the fact that this can be studied in the long records 

 of a single stream, such as the Sudbury, Cochituate, Mystic, 

 Croton or Passaic. Beginning with these streams and working 

 out to the others shown in the table, and also taking into con- 

 sideration the well-known law by which the capacity of the 

 atmosphere to retain moisture is determined by the temperature, 

 we have evolved the following equation to express the relation 

 between evaporation (E.), rain-fall (R.), and temperature : 



E = (ii+.29R)M. 



In this equation M is a factor depending upon the mean tem- 

 perature of the atmosphere.* 



Now, if we take from the table of stream measurements the 

 g'iven rain-fall and temperature for each catchment, and by 

 means of this formula compute the evaporation, that computa- 

 tion will give us what the evaporation should be if it is influenced 

 only by rain-fall and temperature. If the observed evaporation 

 differs from this computed evaporation, the difference may 

 possibly be accounted for by the influence of forests, humidity, 

 wind, or difference in the distribution of rain-fall through the 

 year. We might expect to find that humidity affects the e\'apora- 

 tion to some extent in case this humidity is derived from the sea 

 or other large water-surfaces in the vicinity of the catchment. 

 We are not perpared to say that under these conditions it has 

 not some effect, but the results of our studies given later indicate 

 that this effect is a minor one. It is possible that the evapora- 

 tion on southern stream basins is somewhat decreased by this 

 influence, while on. the Kansas, Desplaines and similar interior 

 streams, it may be relatively increased. 



* Values of M for given temperatures are as follows : 



40°— 0.77, 41°— 0.79, 42°— o 8a, 43°— 0.85, 4(°— 0.88, 45°— 0.91, 46°— 0.91, 47°— 0.97, 48°— 1. 00, 

 49°— 1,03, 50°— 1.07, 51°— 1. 10, 52°— 1. 14, 53°— 1. 18, 54°— 1.22, S5°— 1.26, 56°— 1.30, 57°— 1.34, S8°— 1.39, 

 59°— 1.43, 60° — 1.47, 61° — I. SI. 



