_Fifth._ The party of the second part, his heirs, executors, administrators, or assigns, is to receive from the company and enjoy the benefits of all existing contracts, and shall assure all such contracts and all liabilities of the company accrued or arising therefrom for work done or to be done, and material furnished or to be furnished, for or on account of the road west of the one-hundredth meridian, crediting, however, the party of the first part on this contract all moneys heretofore paid or expended on account thereof. _Sixth._ The party of the second part, for himself, his heirs, executors, administrators, and assigns, stipulates and agrees that the work shall be prosecuted and completed with energy and all possible speed, so as to complete the same at the earliest practicable day, it being understood that the speed of construction and time of completion is the essence of this contract, and at the same time the road to be a first-class road, with equipments; and if the same, in the opinion of the chief engineer, is not so prosecuted, both as regards quality and dispatch, that then the said party of the first part shall and may, through its general agent or other officer detailed for that purpose, take charge of said work and carry the same on at proper cost and expense of the party of the second part. _Seventh._ The grading, bridging, and superstructure to be completed under the supervision of the general agent of the company, to the satisfaction of the chief engineer, and to be of the same character as to the workmanship and materials as in the construction of the road east of the one-hundredth meridian. It is, however, understood that all iron hereafter purchased or contracted for, shall be of the weight of not less than fifty-six pounds to the yard, and to be fish bar joints. _Eighth._ All the expenses of the engineering are to be charged and paid by the party of the second part, except the pay and salary of the chief engineer and consulting engineer, and their immediate assistants, and the expenses of the general survey of the route. _Ninth._ The depot buildings, machine shops, water tanks, and also bridges shall be of the most approved pattern, and they, as well as the kind of masonry and other material used, shall be previously approved by the general agent and chief engineer of the company, and all tunnels shall be arched with brick or stone, when necessary for the protection of the same. _Tenth._ Payments to be made as the work progresses, upon the estimates of the chief engineer, in making which the engineer shall deduct from each section its proportion of the cost of equipment not then furnished, station buildings, superstructure, and cost of telegraph, but all materials delivered or in transit for the account of the company may be estimated for.