being equal, the treatment of a
packet sent by the Open Post is more expensive to the Post Office than
its treatment if sent by Letter Post.

The requirement is imposed in order that compliance with other
conditions may be ensured. In none of the five countries are ordinary
letters allowed to pass at postcard rate if merely enclosed in open
covers. But a printed circular letter, if sent in a sealed cover, would
lose its claim to the privileged rate.

[643] "Fixing a railway rate is, in one word, an art--not a science, and
it is an art which, in Bagehot's phrase, must be exercised 'in a sort of
twilight, ... in an atmosphere of probabilities and of doubt, where
nothing is very clear, where there are some chances for many events,
where there is much to be said for several courses, where, nevertheless,
one course must be determinedly chosen and fixedly adhered to.'"--W. M.
Acworth, _Elements of Railway Economics_, Oxford, 1905, p. 73.

"The problem of railway rates has not, like that of postal charges,
passed beyond the domain of current discussion. This is in part due to
the fact that railways are universally regarded as a source of profit,
to companies when privately owned, to the State when public property;
but it is in larger measure due to the fact that the social significance
of railways is not yet clearly understood. The problem of railway rates
is a problem by itself, and stands as one of the most important of the
unsettled problems of the day."--H. C. Adams, _Science of Finance_, New
York, 1909, p. 280.

[644] "The cost of the service of transport for any given commodity
cannot, under the varying conditions of railway operation, be even
approximately calculated. The first insuperable difficulty is the
division of the expenditure for any given work. Though railway
economists have endeavoured, by means various and ingenious, to allocate
the different items of railway expenditure, they have been unable to
determine such a relatively simple matter as the division between
passenger and goods traffic, and though estimates have been formulated,
many of the charges have been allocated to one head or another by
arbitrary decision, and not as a result of positive
knowledge."--_Railway News_, London, 6th September 1913, p. 396.

[645] "Though all the rates must be so fixed as to pay all the expenses
both of construction and working, separate rates cannot be fixed
according to cost of individual service or even according