The work shall be done under the supervision of the surveyor or engineer appointed by the county board, and when a part, not less than one-fourth of the portion included in any contract, is completed according to the specifications, he shall give the contractor a certificate thereof, showing the proportional amount which the contractor is entitled to be paid according to the terms of the contract. The county clerk shall, upon presentation of such certificate, draw his warrant upon the treasurer for seventy-five percent of said amount, and the treasurer will pay the same out of any funds in the treasury applicable to such purposes; Provided, no proportional amounts shall be certified or paid unless the whole of such contract exceeds two thousand lineal feet.