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Augmentation of the Moon's Hor. Paral-
DIVISIONS OF THE SCIENCE
1. Navigation may be defined as the art of conducting a ship from one port to another, or that science which relates to the determination of the position of a ship at sea. It embraces two distinct methods or branches, which, while absolutely independent of each other, are, in practice, generally carried on together, one serving as a check upon the other.
2. The first of these methods is usually treated under the head of navigation by dead reckoning, or simply navigaţion, and the second under that of nautical assronomy.
3. Navigation by dead reckoning consists mainly in recording the courses and distances sailed by the ship and from data thus obtained determine its position with reference to some known point on land, or to some former position; it also embraces methods of computing the courses and distances to be sailed in order to reach the place of destination. Hence, its execution requires a knowledge of the terrestrial globe and the imaginary lines and circles drawn upon its surface.
The principal instruments used are the charf, representing the part of the sea in which the ship is navigating; the lead, to ascertain the depth and character of bottom; the mariner's compass, and the log, the former to determine the direction
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or course in which the ship is proceeding, and the latter to ascertain the distance run in that direction.
4. Nautical astronomy consists in the measuring of altitudes and zenith distances of celestial bodies and, from data thus obtained, in combination with the recorded time of observation, calculate the position of the ship by the application of astronomical laws and principles. Consequently, this branch of the science requires a knowledge of the different systems of circles imagined to be drawn on the celestial sphere, as well as a familiarity with certain celestial phenomena and the fundamental principles of astronomy.
The principal instruments used are the sextant, the azimuth compass, the chronometer, the nautica/ almanac, and suitable tables to facilitate the solution of the various problems.
5. All the problems appertaining to navigation are solved by plane trigonometry, principally involving the solution of right-angled plane triangles, while those of nautical astronomy are solved by means of spherical trigonometry. For the present we will deal exclusively with that branch of the science defined as navigation by dead reckoning.
THE TERRESTRIAL SPHERE .
FORM AND MAGNITUIDE OF THE EARTH
6. In geography, as well as navigation, the earth is regarded as a sphere. From actual measurements at various parts of its surface it has, however, been found to slightly differ from this, it being somewhat flattened at the poles. This departure from the true spherical form is so trifling and inconsiderable that no practical error can result from treating the earth as a perfect sphere when laying down geographical positions and framing directions for sailing over its surface.
7. The rotundity of the earth is manifested in many ways. The most obvious, and what must particularly strike every mariner, is that when meeting, for instance, a steamer