"The said ships to be each of not less than four thousand tons burden, to be so constructed as to be convertible, at the least possible expense, into war steamers of the first class, and to be built and equipped in accordance with plans to be submitted to and approved by the Secretary of the Navy, and under the superintendence of an officer to be appointed by him, two of said ships to be finished and ready for sea in two and a half years, and the other within three years after the date of the contract, and the whole to be kept up by alterations, repairs, or additions, to be approved by the Secretary of the Navy, so as to be fully equal to the exigencies of the service and the faithful performance of the contract. The said Secretary, at all times, to exercise such control over said ships as may not be inconsistent with the provisions of this act, and especially to have the power to direct, at the expense of the Government, such changes in the machinery and internal arrangements of the ships as he may at any time deem advisable. "Each of said ships to be commanded by an officer of the Navy, who with four Passed Midshipmen to act as watch officers, and any mail agents who may be sent by the Government, shall be accommodated and provided for in a manner suitable to their rank, at the expense of the contractors. Each of said ships, if required by the Secretary, shall receive two guns of heavy calibre, and the men from the United States Navy necessary to serve them, who shall be provided for as aforesaid. In the event of war the Government to have the right to take any or all of said ships for its own exclusive use on payment of the value thereof; such value not exceeding the cost, to be ascertained by appraisers chosen by the Secretary of the Navy and the contractors. "Each of said ships to make four voyages per annum; one shall leave New Orleans every three months; one shall leave Baltimore every three months, touching at Norfolk and Charleston; and one shall leave New York every three months, touching at Savannah; all having liberty to touch at any of the West India Islands; and to proceed thence to Liberia, touching at any of the islands or ports on the coast of Africa; thence to Gibraltar, carrying the Mediterranean mails; thence to Cadiz, or some other Spanish port to be designated by the Secretary of the Navy; thence to Lisbon; thence to Brest, or some other French port to be designated as above; thence, to London, and back to the place of departure, bringing and carrying the mails to and from said ports.