NATURAL HISTORY OF THE BANKS OF THE TAY. 93 
21 St and 28th of last year. The water supply of Hamburg was 
drawn from the Elbe in an unhltered state at a point above the city, 
while Altona received its water supply after filtration through sand 
and gravel from a point below Hamburg. Hamburg, as is well known, 
suffered terribly from cholera during the summer, while Altona was 
almost free of it. Yet in these two cities the soil, the buildings, the 
sewage arrangements, and the population are exactly analogous, the 
only important difference being the water supply. On investigation 
it was found, that one street on the frontier between the two towns 
had the houses on one side supplied with Hamburg water, while the 
houses on the other side were supplied with filtered water from the 
Altona supply. The latter side of the street escaped, while the former 
was attacked by cholera. “ Here, then,” Professor Koch says, “ we 
have to do with a kind of experiment which performed itself on more 
than 100,000 human beings, but which, despite its vast dimension, 
fulfilled all the conditions one requires in an exact and absolutely 
conclusive laboratory experiment. In two large groups of population 
all the factors are the same except one—namely the water supply. 
The group supplied with unfiltered Elbe water suffered severely 
from cholera, that supplied with filtered water very slightly.” In this 
connection the fact that before filtration the Altona water was much 
worse than the Hamburg emphasises the great value of sand and 
gravel filtration as a means of purifying water. There are certain 
precautions, however, absolutely necessary if sand filtration is to be 
thoroughly effective. Koch clearly shows this in his second paper 
referred to above. Periodic outbreaks of enteric fever had occurred 
at Altona from 1886 onwards, and an attack of cholera occurred 
during January and February last, and after very careful bacterio¬ 
logical examination of the water, two or three of the filter-beds were 
found to be acting improperly, due to over pressure of water or freez¬ 
ing of the surface of the filter-beds. 
A second case of the same kind of thing in our own country 
brings the thing nearer home, and is possibly even more to the point. 
In the twenty-first annual report of the Local Government Board, 
published in 1893, there is a most valuable and full report on 
“ Enteric fever in the Tees Valley,” by Dr. Barry. There are two 
periods of six weeks each dealt with in the end of 1890 and begin¬ 
ning of 1891, and the chief towns investigated were Stockton, 
Middlesborough, and Darlington. The water is pumped from the 
Tees, and, after a process of sand and gravel filtration, distributed 
either through the works of the Darlington Corporation, or by those 
of the Stockton and Middlesborough Water Board. 
Before filtration the water is, as in the case of the Tay, polluted 
by sewage run in higher up the stream, especially from Barnard 
