The problem should really be stated thus in terms of cost not of distance. Suppose the roundabout line to be in part or wholly by water, as in competition between the transcontinental roads and the Isthmian or Cape Horn routes, or as between the direct all-rail line from Boston to Nashville, Tenn., and the steamer line from Boston to Savannah, and thence up to Nashville by rail. In such cases the rail lines allow the water routes a differential or constructive mileage in recognition of their relatively cheaper per mile expenses of operation. The differential may sometimes exist, where, judging by the bulk of traffic the advantage, irrespective of the differential, lies with the line giving the lower rate. In other words, as measured by volume of business, the stronger line and not the weaker one enjoys the benefit of the differential. This is the case in the coastwise traffic between Atlantic and Gulf ports; where the bulk of the tonnage goes by steamer and at lower rates than by all-rail lines. The difficult point to settle in all such cases is whether the allowance is made as a voluntary _concession_ to the roundabout line because it _costs_ more to operate; or whether it is a toll or tribute, because, irrespective of the cost, the long line rate is made on the basis of value of service. The problem, then, resolves itself into this: how far in practice does cost of operation really "control" the rate in cases of competition between two lines differently circumstanced in this regard? If cost is of fundamental importance, the "short line," using the term as above said in a figurative sense, "controls" the rate. If cost is an entirely secondary matter, rates being made in accordance with considerations of value of service, the "long line" holds the upper hand, and the short one is at its mercy.