3In the case of an accumulating sinking fund, the interest received in any year from the investment of the sums set apart for the purposes of the sinking fund shall form part of the revenue for that year of the district fund, but the contribution to be made to the sinking fund out of the district fund shall in that year be increased by a sum equal to the interest that would have accrued to the sinking fund during that year if interest had been accumulated therein at the rate per cent. per annum on which the annual payments to the sinking fund are based.