Geography of the Av. 59 
- been as low as 0.09 per centum in a very dry year. In the Mur- 
ray basin the average discharge relative to the rainfall is esti- 
mated to be about 27 per centum from a record of seven years, 
and has risen as high as 36 per centum in a flood year. 
In connection with the regimen of rivers, it appears a proper 
occasion to again refute the popular opinion that the spring and 
summer floods of the Missouri and Mississippi valleys result from 
the melting of the winter snows. This is an erroneous impres- 
sion which I have combatted since 18738, when my duties 
required a study of the floods of the entire Mississippi catchment 
basin. It is only within the last two years, however, that the 
meteorological data has been in such condition that the opinion 
put forth by me could be verified, namely : that the floods of the 
late spring and early summer owe their origin almost entirely to 
the heavy rains immediately before and during the flood period. 
Occasionally a very heavy fall of snow precedes extended gen- 
eral rains ; but in this case the snow is lately fallen and is not 
the winter precipitation. 
Referring to the Missouri valley, the section of the country 
where the winter snowfall has been thought to exercise a domin- 
ating influence in floods, it has elsewhere been shown by me that 
about one-third of the annual precipitation falls over that valley 
during the months of May and June. In either of the months 
named the average precipitation over the Missouri valley, is 
greater than the entire average precipitation for the winter 
months of December, January and February. 
Woiekoff thinks that the anomalies of temperatures shown in 
forest regions, particularly in Brazil—with its abnormally low 
temperatures, are due to heavy forests promoting evaporation, 
aud by causing the prevalence of accompanying fogs thus pre- 
vent more intense insolation. He considers this an argument for 
the maintenance of forests to sustain humidity and distribute 
rain over adjacent cultivated land, as well as to maintain the fer- 
tility of the soil, which diminishes rapidly by washing away of 
-the soil after deforestation. 
W. Koppen has devised a formula for deriving the true daily 
temperature from 8 4. M., 2 Pp. mM. and 8 Pp. M. observations in con- 
nection with the minimum temperature, in which the minimum 
has a variable weight dependent on place and month. The 
results of Koppen’s formula tested on six stations in widely dif- 
ferent latitudes, indicate that it is of value. 
