45 
filtration plant in 1906. from 55 to 86 per cent of the total bacteria in 
the Potomac Kiver water are removed by storage in the three sedimen- 
tation reservoirs before the water is applied to the filter beds. These 
storage reservoirs have been in operation since 1902. During the two 
or three }^ears prior to the installation of the filters no systematic 
bacteriologic examination of the water was made : so we do not know 
what was the bacterial content of the water supplied to AYashington 
during that period and can not accurately compare it with that of the 
water after filtration was begun. Y"e have no accurate data on which 
to base a comparison of the amount of typhoid fever infected excreta 
poured into the Potomac Eiver in 1906 with that of previous years. 
AYe know practically nothing of the condition of symbiosis, etc., under 
which the typhoid bacillus will live best in a river water during cer- 
tain seasons and perhaps certain years. 
The quite regular seasonal (summer) prevalence of typhoid fever 
in Washington, together with the absence of frequent winter and 
spring outbreaks, is regarded by some as good evidence that the 
Potomac Kiver water has played but a minor role in transmitting the 
infection. In Albany and other cities where contaminated water is 
known to have played a prominent role, improvement in the water 
suppl}' not only diminished the total amount of typhoid fever, but 
changed the character of the typhoid curve. The rule is that where 
there is much water-borne typhoid there is also comparatively much 
winter and spring typhoid. In Albany the filtration of the water 
markedly reduced the prevalence of typhoid fever throughout the 
entire year, but most markedly during the winter and spring, during 
which seasons there was, prior to filtration, relatively much more 
typhoid than during the summer season. On the other hand, Y^ash- 
ington has had relatively little winter and spring typhoid ; the annual 
curve, even Avhen excessively high and before any attempted purifica- 
tion of the water supply, shows a marked summer maximum or the 
so-called “ normal ” curve. The lYashington curve has changed 
markedly in height, but little in character. 
In the majority of cities supplied with a polluted water and having 
a high typhoid rate in the winter and spring, the pollution of the 
water has been nearer the intake and presumably much more con- 
centrated than is the case of Y'ashington’s water supply. The vast 
bulk of the pollution of the Potomac occurs at points over 100 miles 
above the intake of the water supply for lYashington, and little or 
practically no direct pollution takes place within 19 miles of the 
intake. Therefore, as the dilution of the sewage must be tremendous 
by the time it reaches the intake at Great Falls it may be that the 
small amount of typhoid infection finding its way into the river 
from the relatively small number of cases occurring on the watershed 
during the winter and early spring is not sufficient to affect markedly 
