Appraisals of the land and timber thereon shall be made, in the manner prescribed in , not less frequently than once in each ten-year period, and the amounts due hereunder in any year shall be computed as specified in upon the basis of the last appraisement. The expenses of making the appraisements provided for in this subchapter shall be paid by the Secretary of the Treasury upon certification by the Secretary of the Interior, from that portion of the receipts derived from such lands and timber payable to the counties and shall be deducted from any amount due said counties.section 2621 of this titlesection 2621 of this title