Thus one arrives at the general conclusion that approximately two-thirds of the total expenditure of a railroad and more than one-half of the actual operating expenses are independent of the volume of traffic. The remaining third of all expenditures, or what amounts to the same thing, the other half of the operating expenses, are immediately responsive to any variation in business. Applied to the railroad net of the United States, this means that only about one-third of the $2,000,000,000 disbursed in 1905--an amount equal to about two and one-half times the national debt--was susceptible of variation according as the traffic expanded or decreased. This provisional estimate, defective principally because of inadequacy of the returns as to depreciation and replacement, agrees in the main with computations based upon other data. The Vice-President of the Southern Pacific Railroad, in 1892, after extended investigation, arrived at precisely the same general conclusion. The great German authority, Sax, estimates that one-half of a road's operating outlay is constant and that this operating outgo equals about half the total expenditure, the other half being capital cost and hence constant. This calculation places the constant factors even higher than ours, viz., at about three-fourths of the total expenditure. Eaton states that half of the operating expenses respond to changes in the volume of traffic. Our estimate, above detailed, seems to be in accord therefore with good authority, and differs but little from any of the reliable writers.