The advantage to the railway conducted under private management may be defined to be the excess of receipts from the traffic over the out-of-pocket expenses actually incurred in handling the traffic. To obtain this maximum it has been found necessary to vary the charge according to the nature of the goods. Elaborate, detailed classifications of goods have been arranged with distinct scales of rates for each class, devised on the basis of charging each kind of goods with the rate likely to yield to the railway the maximum of advantage as defined above.[646] Although somewhat crude and a little empirical, certainly largely arbitrary, this method has been almost universally adopted for the determination of railway charges.[647] A characteristic feature of such charges is that account is invariably taken of the distance over which the goods are transported. In contrast with this, the principle of uniformity of rate irrespective of distance has been universally adopted in regard to all postal packets other than parcels, and to some extent for parcels. The application of the principle to parcels rests, however, on other grounds than its application to letters. Sir Rowland Hill himself never contemplated that the principle was necessarily applicable to all matter which might be sent by post.[648] The circumstances under which he made his discovery, and the facts on which he relied, make it plain that, in the absence of other overpowering considerations, the grounds advanced in the case of light letters will not justify uniformity of rate irrespective of distance for packets of considerable weight, which necessarily involve appreciable cost for transportation. From the financial point of view, the uniform rate is, moreover, inapplicable to any class of traffic not secured to the Post Office by monopoly, since private undertakings will always step in and take away the profitable sections. For heavy parcels a uniform rate cannot be justified. There are, however, certain considerations not purely economic which may be held to justify a uniform rate for small parcels, especially if it be held that the State may conduct such a business for the advantage of the public, and abandon to some extent ordinary commercial balancing of cost and revenue.