#Tides and currents.# Information must be obtained as to the movement of the water, both vertical and horizontal. The rise and fall of the tide are obtained by tide gauges, either automatic, which draw a continuous tidal curve on a roll of paper, or simple tide staffs, which must be read at intervals. The currents, whether due to the tides or other movements, are measured by noting the movement of partially submerged floats. Less accurate but useful information as to currents is obtained from the logs of vessels. [Illustration: FIG. 18. DRAGGING FOR DANGERS WITH A LONG WIRE.] #Dragging for dangers# has long been resorted to for the investigation of isolated spots. A valuable and successful means has been employed recently of making sure that an area is free from shoals or rocks having less than a certain depth. This is done by dragging through the water a wire from 500 to 1400 feet long, and suspended at the required depth, with suitable buoys and weights, and kept taut by the angle of pull. If, for instance, the wire is set at a depth of 30 feet it will indicate the presence of any obstruction of less depth by catching on it and upsetting the buoys, and such spots are at once marked and investigated. Considerable work has been done with such drags in the last few years on the Atlantic and Gulf coasts and on the Great Lakes. This is of course a somewhat tedious process and gives no information as to depths greater than that for which the wire is set, but the experience already had indicates its great value. It will probably be found desirable in time to thus drag all water areas important to navigation where the depth is near the draft of vessels and the irregular nature of the bottom gives indication of dangers. In extensive dragging operations near Key West and in Jericho Bay, Maine, a number of shoals have been picked up which were not found in the original surveys. A remarkable instance of the value of the drag was the recent discovery of a rock in Blue Hill Bay on the coast of Maine. This rock has but 7 feet of water over it, and is only 6 feet in diameter at the top. It is surrounded by depths of 78 feet, from which it rises nearly perpendicularly. The original survey gave no indication of a danger here, and its existence was not suspected until it was discovered with the wire drag. Another method of dragging that has been employed is by means of a pipe suspended beneath a ship's bottom. #Magnetic variation.# As the compass is a universal navigational instrument, information as to the magnetic variation is needed for the charts. The angle between the direction of the magnetic needle and the true north is measured at various points on both land and sea, and at some stations these observations are repeated after a number of years. From these results magnetic maps are made, from which both the variation and its annual change may be taken.