The simplest method of obtaining latitude, and the one most eo»nnonly used, is what is called "latitude by meridian altitude.'' This is taken at the time the sun bears north or south as it crosses the meridian. The observer knows the approximate longitude. A short time before the body reaches the meridian, he begins to take altitudes of it (usually the sun is taken). As soon as the altitude slims increasing and begins to decrease he knows that the sun has crossed the meridian and that the maximum altitude ob tallied is the one sought. If h is the altitude thus obtained and (/ is the declination of the body obtained from the Nautical Almanac, then the latitude = 90° — (11 +(I), the sign of d hieing plus if the body is on the same side of tine equator as the ship and minus if on the opposite side. If a meridian altitude cannot be obtained. a spherical triangle must he solved, in which ease the time must be as near noon as possible, other wise a considerable error in latitude may result from a small one in observation or tittle.
The ship's position when ascertained is plotted (i.e. correctly located) on the proper chart and a line drawn connecting it with the preceding po sition. The continuous line joining the various positions is called the ship's track and shows her and longitude at the different parts of her voyage. As soon as a position is determined
and placed upon the chart the navigator deter mines the course for the next day, or next few hours as the ease may lie, by finding the course which must he steered to arrive at the point de sired. This operation is faeilitated by the out line drawing of a compass placed on the chart, the circumference of which is divided into de grees. By means of parallel rulers the direction of the new course is transferred to this compass and the correct course ascertained. The course thus obtained is either the true course or may netic course, according to the character of the chart compass, some of which are laid down with the north point directed toward the true north and some with it directed toward magnetic north.
If the log-book of tine ship is carefully kept and the speeds and courses accurately determined, the navigator may each day ffitermine not only his position, and the course and run since the previous day, but also the ocean currents, because —neglecting errors in observations, etc.—the dif ferenee between the course and run by Mg (or the dead reckoning) and the actual course and rim as determined by astronomical observations is due to current. He also frequently checks the table of compass errors by bearings of the sun —the correct directions of the sun hieing com puted.