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out that the vast majority of the most dangerous wells in the City are so situated
that sun-light cannot penetrate to them; and except in the case of the Parsis,
the view appears to be based rather upon some vague idea that air and day-
light will purfiy water, than on any strict religious grounds. It is a common
experience to find that Indian servants employed in large residential flats in
the City, which usually have many closed cisterns upon their terrace roofs,
persist in opening the lids of the cisterns, in order that the sun may gain access
to the water.
400. As regards the suggestion that the filling in of tanks and wells
may cause a rise in the level of the subsoil water, or that the covering of wells
may interfere with their ventilation, it may be pointed out that-
(1)  The filling in of all the tanks and wells on the island would not affect
the subsoil water level to any appreciable extent. The amount of water
present in the wells and tanks represents only a fractional proportion of that
present in the soil; and the water level of wells or tanks is usually much below
the level of the subsoil water. The average rainfall in Bombay is close upon
72 inches or nearly 6 feet of rain per annum which is equivalent to a fall of
37·5 gallons of water upon every square foot of the surface of the land. Much
of this enormous volume of water drains away into the sea, but a considerable
amount which would otherwise drain away goes into the wells and tanks and
raises their water level considerably; and the existence of these open wells and
tanks helps to maintain the subsoil water at a higher level than it would
otherwise reach.
(2)  As regards the supposed necessity for ventilating wells, it may be
pointed out that the fact that a well is quite open and uncovered does not
necessarily ensure its ventilation. The only way to ventilate many wells is by
drawing out a considerable volume of water from them, and a well entirely
covered may be better ventilated, if water is being continually pumped out
from it, than a well the mouth of which is quite open. Other things being
equal the water from a well that is entirely covered will be cleaner and much
purer than that from an open well, especially one in the midst of a populous
city, exposed to dust and all sorts of contamination. At the present time
very few of the wells in Bombay supply water that is fit for potable use; and
the covering of the majority of these wells would be a great sanitary improve-
ment even apart from the question of malaria prevention.