CHAPTER V. 50. Ancient Engineering Science 60 51. Ancient Views of the Physical Properties of Materials 61 52. Roman Civil Engineers Searching for Water 62 53. Locating and Designing Conduits 63 54. Siphons 64 55. Healthful Sites for Cities 65 56. Foundations of Structures 65 57. Pozzuolana and Sand 66 58. Lime Mortar 66 59. Roman Bricks according to Vitruvius 66 60. Roman Timber 67 61. The Rules of Vitruvius for Harbors 67 62. The Thrusts of Arches and Earth; Retaining-walls and Pavements 68 63. The Professional Spirit of Vitruvius 68 64. Mechanical Appliances of the Ancients 69 65. Unlimited Forces and Time 69 PART II. _BRIDGES._ CHAPTER VI. 66. Introductory 70 67. First Cast-iron Arch 70 68. Early Timber Bridges in America 71 69. Town Lattice Bridge 72 70. Howe Truss 74 71. Pratt Truss 76 72. Squire Whipple’s Work 77 73. Character of Work of Early Builders 77