The strong tidal stream in the southern portion of the North Sea renders navigation rather anxious work in misty weather, or in darkness, especially as the normal rate of the current is much influenced by weather. Naturally, there is less danger of hitting a shoal which one is endeavouring to avoid, if the ship is steaming either directly with or against the current; the error in such cases is confined to the time at which any particular position will be reached. But when steaming _across_ the current a small eccentricity on the part of the latter may make all the difference between reaching the desired position and missing it altogether. The current running parallel to the Belgian coast attains a speed of about three knots under normal conditions. Should a three-knot allowance be made when steaming across the tide there would be a serious error of position at the end of an hour's run if the tidal stream happened to be running at the rate of two and three-quarters or three and one-quarter knots; it must be realised that a ship cannot discover the rate of the tidal stream in a given area until she has completed her passage through it; i.e., until she has already suffered from its eccentricities. For the purposes of the particular case under review {93} it was necessary for the expedition to arrive at an _exact_ position, thus tidal calculations and navigational aids assumed great importance. But they alone were insufficient to ensure accurate navigation. Compasses must be correct, or their errors known, and the speed of the ship due to its own engines requires to be carefully gauged. For a given speed of engine the speed of a ship varies according to her draught and the state of the ship's hull under water. All these considerations will serve to show the extreme necessity for working out courses, speeds, and times beforehand with the utmost accuracy, for repeatedly checking them to ensure the absence of clerical error, for reconsidering allowances for the vagaries of the elements, and for correcting the results from day to day according to the tidal differences due to the ever-changing phases of the moon. The careful navigator always follows a similar procedure, but, in ordinary cases, he knows that a fault in position can probably be remedied in time to avoid untoward incident. In our movements, however, there would be small chance of remedy if the blockships failed to find the canal entrance, or if the submarines were unable to locate the viaduct, or if the storming vessels missed the Mole. [Sidenote: Clearing the Mines]