For preliminary examinations, and for all purposes in which great accuracy is not required, the little instrument shown in Fig. 9,—"Wells’ Clinometer,"—is exceedingly simple and convenient. Its essential parts are a flat side, or base, on which it stands, and a hollow disk just half filled with some heavy liquid. The glass face of the disk is surrounded by a graduated scale that marks the angle at which the surface of the liquid stands, with reference to the flat base. The line 0.——0. being parallel to the base, when the liquid stands on that line, the flat side is horizontal; the line 90.——90. being perpendicular to the base, when the liquid stands on that line, the flat side is perpendicular or _plumb_. In like manner, the intervening angles are marked, and, by the aid of the following tables, the instrument indicates the rate of fall per hundred feet of horizontal measurement, and per hundred feet measured upon the sloping line.(6) Table No. 1 shows the rise of the slope for 100 feet of the horizontal measurement. Example: If the horizontal distance is 100 feet, and the slope is at an angle of 15°, the rise will be 17-633/1000 feet. Table No. 2 shows the rise of the slope for 100 feet of its own length. If the sloping line, (at an angle of 15°,) is 100 feet long, it rises 25.882 feet. TABLE No. 1. DEG. FEET. 5 8.749 10 17.663 15 26.795 20 36.397 25 46.631 30 57.735 35 70.021 40 83.910 45 100.— 50 119.175 55 142.815 60 173.205 65 214.451 70 274.748 75 373.205 80 567.128 85 1143.01 TABLE No. 2 DEG. FEET. 5 8.716 10 17.365 15 25.882 20 34.202 25 42.262 30 50.— 35 57.358 40 64.279 45 70.711 50 76.604 55 81.915 60 86.602 65 90.631 70 93.969 75 96.593 80 98.481 85 99.619 With the maps before him, showing the surface features of the field, and the position of the under-ground rock, the drainer will have to consider the following points: 1. Where, and at what depth, shall the outlet be placed? 2. What shall be the location, the length and the depth of the main drain? 3. What subsidiary mains,—or collecting drains,—shall connect the minor valleys with the main? 4. What may best be done to collect the water of large springs and carry it away? 5. What provision is necessary to collect the water that flows over the surface of out-cropping rock, or along springy lines on side hills or under banks? 6. What should be the depth, the distance apart, the direction, and the rate of _fall_, of the lateral drains? 7. What kind and sizes of tile should be used to form the conduits? 8. What provision should be made to prevent the obstruction of the drains, by an accumulation of silt or sand, which may enter the tiles immediately after they are laid, and before the earth becomes compacted about them; and from the entrance of vermin?