The mention of the sinking fund leads naturally, however, to a reference to the provisions of the Thurman bill relating to sinking fund investments. Mr. Thurman proposed in 1878 that the sums credited to the Pacific railroads’ sinking funds be used to purchase United States bonds, preferably 5 per cent bonds because other outstanding issues were either insufficient in amount or had only a short time to run. Up to June 30, 1897, about $6,000,000 were available for such purchases. But this limitation of the field of investment seriously crippled the earning power of the fund by requiring the purchase of securities of classes which either bore low rates of interest or which commanded considerable premiums in the market. The average premium paid by the Central Pacific up to 1883 was approximately 13 per cent.[534] In 1891 the Commissioner of Railroads reported that between the date of the creation of the sinking fund in 1878 and the date of his report, on June 30, 1891, the government had bought bonds with a par value of $6,138,800 for the Central Pacific, for which it had paid a premium of $1,110,409.62, or an average of 18 per cent. At times the premium paid had gone as high as 35 per cent,[535] and in the earlier years the payments on account of premiums materially exceeded the earnings of the sinking fund in the way of interest. This excess disappeared, of course, as the fund grew larger, but the absolute amount of the premium continued to grow. The principal bonds in which the sinking funds were invested up to 1882 were the United States currency sixes, the 5 per cent funded loan of 1881, and the 4 per cent funded loan of 1907. In 1881 the funded fives matured and were continued at 3½ per cent. In 1882 the Treasurer of the United States exchanged these bonds for a new 3 per cent issue. Inasmuch as the bonds which bore the higher interest rates all commanded a premium, the actual yield of the fund up to 1886 was only from 2½ to 3 per cent. This condition was recognized as disadvantageous by all concerned. The Commissioner of Railroads declared in 1883 that it would require a century or more at the rate provided in the Thurman Act to accumulate a fund sufficient to discharge the railroad debt, with a strong probability that even then it could not be done.[536] The Auditor of Railroads in 1879, the Secretary of the Treasury in 1881, and the Commissioner of Railroads, in various reports, all urged that the field for investment of the sinking funds be widened, at least to include the first mortgage bonds of the Pacific railroads. Since the lien of these bonds was prior to that of the sinking fund itself, it seemed appropriate to allow the Secretary of the Treasury to buy them with sinking fund money. The suggestion was adopted by Congress in 1887,[537] with the result that interest on the funds placed in this new investment amounted to 4.15 per cent. This was a substantial increase from the 2½ or 3 per cent realized from government bonds,