92
Water that has been in pipes.—Very little need be said on this head. As we are
not dealing with forces that are strictly natural, it is very difficult to generalize from
these figures. The only thing that can be said. is that pipes have a sort of general
levelling tendency on the organisms ; that is to say the figures show that great
inequalities of percentage tended to disappear. Two important facts have been
demonstrated by our work :—
(i) That silt in pipes has a very serious action in spoiling the quality of the
water ; that it will go on adding organisms to water for a very long time. Examples
of this are seen in Madras, and in the Marlimund pipe line in Ootacamund. Fortu-
nately in some cases the silt is easily removed. if the system be fitted with wash-out
valves and the pressure is sufficient.
(ii) The action of clean pipes in a water has been demonstrated. by Dr. S..
Delepine (Journal of State Medicine 1898) his conclusions are :—
" The purification is sufficiently rapid to have made it possible to observe-
appreciable change in the bacterial contents of good drinking water during its slow
passage through a system of small vessels and tubes, the total. length of which did
not exceed 150 cm. (50 cm. only being occupied by vessels of large diameter), in
which the rate of flow varied from 3 cm. to 20 cm. per hour.
" The greatest amount of purification occurred when the rate of flow in the above
apparatus was about S cm. per hour. When the rate fell below 4 cm. there was an
increase instead of a reduction in the number of bacteria."
Although it is not possible to estimate the rate of flow in the mains in this
country we have many examples of the purifying action of pipes on a water-supply.
Table IV (d) shows that the average of the pipe water has -undergone an
improvement on the filtered waters. A reference to the subsequent analyses in Part II
demonstrates the beneficial action of clean pipes, in removing fœcal bacilli from water.
Conjeeveram, Nellcre, Ootacamund and Kurnool.
Best quarter.—From table XIII (d) it will be observed that the worst quarter
as regards the purity of water in this Presidency is July, August and September.
There is considerable improvement during the next quarter, namely, October, November
and. December in rivers, lakes and springs, but the best water appears to have been
obtained in the third quarter. The remarkable scarcity of coli communis in this
quarter is very noteworthy. At this time of the year there is usually a fair abundance
of water. As water begins to be Scarce the quality usually falls off.
A reference to Chapter XV will show also that cholera, dysentery and diarrhœa
are less prevalent during the months of December, January and February. Hence
it seems likely that the quality of water supplied has a very considerable influence
upon these diseases.
A word may be said on the advantages and disadvantages of various sources of
water as applied to this country
(1) Rivers.—In the Madras Presidency we have an arrangement which is not in
use in other parts of India and which merits a brief description. This is what we know
as an infiltration gallery. Many of the rivers in this Presidency have a bed which
is made up of extremely coarse sand ; this is derived from the disintegrated granite
and gneiss in the hills. This coarse material is the one of the necessary factors for the
satisfactory working of an infiltration gallery ; a fine sand has a tendency to choke up
the gallery. The arrangement is constructed as follows :—In the bed of a river,
which, as a rule in this country, is dry for a considerable number of months in the
year, a trench of ebnsiderable length, four or five hundred feet long, is dug. The
depth, usually from twelve to thirty feet, varies according to circumstances, but it
must reach down to the layer of water bearing sand. In this two rows of twelve or
eighteen inch, glazed earthenware pipes, are laid end to end, the joints being left open.
Around the pipes on all sides are placed first large stones, then gravel to a thickness of
about five feet in order to prevent the sand being sucked into these pipes. The river
sand is then replaced and the pipes are connected up with the pumping well of the
engine and the water drawn out and supplied to the town. The action of such gallery
is two-fold—(1) it is exactly similar to a well placed in the river, and taps any subsoil
stream that exists in the sand of the river bed. In many of the rivers in this country