Whenever it shall become, in the opinion of the Secretary of the Army, necessary or desirable to hire a dredging plant or plants for the performance of any of the public work carried on under his direction the said Secretary may, in his discretion, agree for the same, either in the manner customary on , or on the basis of an equitable reimbursement for deterioration of plant when in use by the Government, and a reasonable percentage of the total cost of the work.March 2, 19071907-03-02