390 
Eeport of the Chemist of the 
"ITT A rnTTIT) L'T 1 17 T\ 
Years 
of 
record. 
Mean annual 
rainfall, inches. 
Mean annual 
discharge, 
inches. 
Difference or 
evaporation, 
inches. 
4y . 11 
OO AO 
2Z . 08 
27. Uo 
6 
46.10 
21.93 
24.17 
Mystic river 
5 
42.78 
19.65 
23.13 
Croton river 
13 
46.64 
26.35 
20.29 
Professor G-eo. F. Swain* says: " It has been found by the expe- 
rience of cities in regard to their water supply and by continued 
gauging of streams, that in very dry years in this climate, the total 
amount of rainfall discharged by streams falls as low as from twelve 
to fifteen inches over the entire drainage basin." He also states from 
data derived from the Smithsonian tables that the minimum rainfall 
is about seven-tenths of the mean. The average rainfall for the section 
under consideration is over forty inches, and the minimum rainfall is 
rarely below thirty-five inches. Calculations based upon these figures, 
and upon the assumed discharge of a large number of streams, indi- 
cate that throughout the Eastern and Middle States the evaporation 
for large areas, except in rocky and mountainous regions, is scarcely 
ever less than twenty inches, and in most localities considerably 
exceeds this. 
What the effect of a diminished rainfall for a series of years would 
be is not evident; it is quite certain, however, that it would diminish 
the amount of water discharged by streams to a greater extent than it 
would diminish the evaporation. This is well illustrated by results 
obtained on streams from which the water supply of Boston is 
derived, for the years 1879 and 1880, which were exceptionally dry. 
In the following table is given the rainfall and amount of water 
discharged for these two years for the basins of Lake Cochituate, 
Sudbury and Mystic rivers : 
* Report of Water-Power of Eastern New England, Tenth United States 
Census, Vol. XVI. 
