All sealed proposals for contracts shall be for a sum certain as to the price to be paid or received and no proposition, which is not thus definite and certain, or which ;

contains any alternative, condition, or limitation as to price, shall be received or acted upon.

21. No more than one proposition shall be received from anyone person for the same contract, and all the propositions of the person offering more than one, shall be rejected.

22. Every person who shall enter into any contract for the supply of materials, or the performance of labor, shall give satisfactory security to the Commissioners for the faithful performance of his contract according to its terms.

23. All materials procured or partially procured, under a contract with the Commissioners, shall be exempt from execution but it shall be the duty of the Commissioners ;

to pay the moneys due for such materials* to the judgment creditor of the contractor, under whose execution such materials might otherwise have been sold, upon his producing to them due proof that his execution would have so attached, and such payment shall be held a valid payment on the contract.

The Common Council shall authorize the Commissioners to draw upon the 24. Comptroller of the city for any sum in favor of, and to be paid to, the owner of any lands, water streams, or property acquired by virtiie of this act, and in favor of, and to be paid to, any contractor, for any sum due upon his contract, and also for their incidental expenses. Such drafts shall specify the objects for which they are drawn, in the manner provided in the seventeenth section of this act, as nearly as may be and the Common Council shall ;