Inconsistent as such a principle is with any kind of classification of goods, and leading to the consequence that a rate might be the same for a bale of cotton as for high priced silks, its effect might be to revolutionize trade. But there is a preliminary difficulty; how is the cost of conveyance to be ascertained with anything like accuracy? How is the cost of conveying a particular consignment or even the average cost of every kind of traffic to be found? What the transit of full loads of coal in this country, or of grain in America, from point A to point B costs may be approximately found. Allowances may be made for the maintenance of the permanent way, for cost of engine power, and the wages of drivers, guards, &c.; and calculations, more or less accurate, can be made as to the cost of conveyance even over lines of varying gradients. The solution even of this simple form of problem would be difficult. When in cases before the Railway Commissioners it has been attempted to discover the _actual_ cost of conveying a particular kind of traffic, the operation has been laborious. The companies interested have been compelled to incur great expense in procuring returns and information, and the result has in general been only approximately accurate. Very complex and difficult is the real problem. A large portion of the traffic of the country is carried in trains which pick up and set down wagons at intermediate stations. In the same truck may be goods of all classes and different quality or bulk for different destinations. One article of great bulk and light weight may be carried in a truck by itself or along with articles of great weight and small bulk.[4] There is a further difficulty in the fact that, while certain fixed expenses remain much the same, no matter what may be the volume of traffic, the movement or operating expenses increase with the traffic. It may be confidently stated that no trustworthy data as to the cost of conveying each consignment or each class of goods in the actual intricacy of business could be obtained. At best only estimates could be roughly arrived at by arbitrarily making allowances and assumptions. Will those who talk about cost of service reveal the formula by which they can accurately calculate the cost of carriage of a particular article carried in the same truck with a dozen others, all coming from different places and destined for different stations over three or four different lines, the cost of no two of which has been the same, and the working expenses of which are totally dissimilar? If they have discovered this formula, it remains to be stated how it may be applied. [4] NOTE.--See illustration of cubical contents in proportion to weight, page 83.