While the compass gives indispensable information as to direction, and is constantly under the eye of the pilot, it of course can give no direct information as to the distance traversed by the ship, and hence does not by itself suffice to tell the navigator his whereabouts. In the early days there was indeed an expectation that the observed declination of the compass would reveal to the navigator his longitude and that the observation of the dip might enable him to determine his latitude. But more extended observation shows that this was asking altogether too much of the compass, and while it may be useful as an accessory it is by no means the navigator's chief reliance in determining his location. This is accomplished, as everyone is aware, in clear weather by the observation of the heavenly bodies. In cloudy weather, however, such observations obviously cannot be made, and the seaman must direct his ship and estimate his location--an all important matter when he is approaching the coast--by what is called dead reckoning. One element of this reckoning is furnished by the compass, inasmuch as that is his sole guide in determining the direction of the ship's progress. The other element is supplied by the log which furnishes him a clue as to the distance traversed hour by hour. It is rather startling to reflect that the navigators of the middle ages had no means whatever of determining the rate of progress of a ship at sea, beyond the crudest guesses unaided by instrument of any kind. When Columbus made his voyage he had no means of knowing what distance he had actually sailed; nor was any method of measuring the ship's speed utilized throughout the course of the ensuing century. In the year 1570, however, one Humfray Cole suggested a theoretical means of measuring the ship's rate of progress by means of an object dropped back of the ship and allowed to drag through the water; and this suggestion led a generation later to the introduction of the log, which was first actually tested, so far as can be learned, in the year 1607.