From the position thus obtained he shapes his course in the direction best calculated to reach his destination; that is, if the way in which the wind is blowing will allow him to do so (in a sailing ship). This is done by bringing the desired point of the compass in a line with a mark drawn upon the side of the round box in which the compass swings, which mark really represents the ship's head. And if, as is popularly supposed, the compass needle always pointed true to the north, navigation would be very simple. But, alas! this instrument is full of vagaries. Apart altogether from such harassing complications as the attraction of the iron in the ship produces, there is the variation of the compass itself from the north, which changes continually as the vessel goes on her way. Then there is bad steering, and, worse still, the effect of unknown currents, which sweep the ship away in some direction which cannot be calculated until after it has occurred. The speed of the ship is known by the use of a beautiful instrument, called a patent log, which, towed behind the ship, registers her rate of progress with an accuracy unobtainable by any cyclometer. Where, for economical reasons, the patent log is not used, the mariner must rely upon a primitive instrument, called a "logship," which, being used once every hour or two hours, cannot, however good it may be, give such true results as the patent log, which records every foot of the distance travelled. When, however, the heavenly bodies, which are always faithful and reliable, are obscured by bad weather, and the master has to depend upon a position obtained by a calculation of the course made by compass and the distance run by log, he may well be uneasy if he be in difficult waters near land. For the compass can only be corrected by the aid of the sun, moon, or stars when at sea, and if _they_ are invisible it may be a very unsafe guide, although an indispensable one. Roughly, these are the principles upon which a ship is navigated, modifications and extensions of which go to make up the perfect navigator. And no matter how perfect a navigator a master may be, he will always, if he be wise, see that the officers work out the ship's position independently, so that a comparison may be made between the various workings, and any errors detected.