would
result in a comparatively small reduction of the number of letters.[638]
But moderate variations of postage on ordinary letters are difficult to
make, since popular charges, such as a penny or halfpenny, while they
offer obvious advantages from many points of view, are not susceptible
of slight modifications.

The variation of rate according to the weight of the packet is a point
which has received insufficient attention. There can be no doubt that
the cost to the Post Office of performing the service it affords in
respect of packets of any kind entrusted to it increases with the
increase of the weight and size. But it does not increase
proportionately. A letter of 8 ounces does not cost twice as much to
collect, transmit, and deliver as a letter of 4 ounces. The operations
of stamping, sorting, and making up for despatch occupy more time and
cause more inconvenience in the case of the larger packet, but the
difference is slight when compared with the difference in size and
weight. Nor does the cost of conveyance vary directly with the weight.
In any system of rates, therefore, which are accurately adjusted to the
cost of the service, the rate of charge must increase considerably less
rapidly than the increase in weight, that is to say, the rate would be
degressive. Of modern postage rates very few are constructed on this
principle, and to that extent they are uneconomic. In the case of
letters, since the weight of the packet is normally unimportant, and
simplicity of charge very important, this factor has been for the most
part ignored.[639]

* * * * *

The same consideration which makes the uniform rate irrespective of
distance economically just in the case of ordinary letters, takes away
any ground on the score of cost of service for a special rate for local
letters lower than the general uniform rate. On the other hand, the
considerations which make for monopoly and unified control in the case
of a general service, do not apply with the same force in the case of a
service limited to a small area. In the latter case, competition can
much more easily be set up; and as the uniform penny rate is much higher
than the cost of service even in the case of long-distance letters,
competing agencies, which can leave aside unprofitable districts, such
as the rural districts, can secure a profit on a local service while
charging much lower rates. The maintenance of a local rate for letters