the line H K is equal to H G—that is, our present distance from G is the number of miles we have made while sailing from K to H, allowance having, of course, been made for the set and drift of the current.
As a rule, cross-bearings taken in this way are not nearly so reliable as those taken of two objects from one position, in the manner already explained; for unless we have an accurate knowledge of the local currents, we can only approximately calculate the true distance and course made by the vessel in the interval between taking the two bearings. But in the present case, as we happen to be sailing slowly and are in shallow water, we are enabled by employing what is termed the ground-log to gauge with great precision the effect of any stream that may be running, and to lay down our exact position on the chart. To effect this we attach our ordinary log-line to our hand-lead and heave it over the stern. We then reel out the line and employ the log-glass just as we would do when heaving the log-ship; but the lead, instead of being suspended in the water, as is the log-ship during the operation, remains fixed on the bottom; and therefore, when we read off the knots we are making, we ascertain not our rate through the water, as is the case when we employ the log-ship, but our speed over the ground. The direction in which the line leads over the stern indicates the direction in which we are moving over the ground, forming an
