270 ECONOMIC PRINCIPLES

questions relating to the construction of the building, the
neighbours, or the entire district surrounding it, would be
liable to be affected by any serious defect in construction
likely to constitute it a centre at which infectious disease
might develop, and from which it might be propagated
among innocent persons. To all intents and purposes these
become parties to the contract, and may claim to insert in it
provisions securing them from injury, and to exercise supervision,
 as to the protection of their interest, through the local
government authorities of their district.
One further point may also be illustrated by reference to
this same example. There are not a few cases in which
a society may benefit if all its members follow a certain
line of action, while such action on the part of isolated
members of the society, the rest neglecting to co-operate,
would not yield sufficient advantage to justify the expense
or trouble involved. The cleansing of city streets is a case
in point. If each of the inhabitants of a street cleanses the
portion in front of his premises, the whole street gains at a
trifling cost to each. If only one here and there takes such
action, the neglect of his neighbours almost nullifies the
advantage resulting from his effort or outlay. An ordinance,
compelling each to care for his own share of the work, would
secure to each sufficient advantage to warrant his outlay,
if all the others were compelled to do their shares, But the
same end may be attained, perhaps at less cost and with
greater efficiency, if each contributes to the cost of the
necessary cleansing, and the cleansing is carried out by
contract or by a department of the local government. Since
the loss to an individual who neglected to do his share of
the work, or cause it to be done, would be small in case his
neighbours all attended to their shares, compulsion may be
necessary to secure to all the advantages of uniform action.
This last instance illustrates a considerable class of cases in
which the advantage of the community is best secured by
common action through a government agency. When each
would gain from a certain line of expenditure more than the
equivalent of his share of the cost, while none would gain
50 much as to make it worth their while to undertake the