SAILINGS; or THE SAILINGS. The various modes;in which the calculations of a ships true course are conducted, have already, under the ; head Recgoeleus .vv SEA, been referred to individually, as forming part of a seamen's " day's work," or dead reckoning as it is called ; distinctive of such portions of hie work as depend ou observa tion of the heavenly bodies. In this article it is proposed to review the peculiarities of the 'tailings with reference to the great problem of the accurate navigation of the surface of a sphere.
Under the word NAVIGATION the subject has been treated rather as the practice of a general system ; and its details hero been partly re ferred to under the terms Con PASS, GREAT-CIRCLE OR TANGENT SAILING, LONGITUDE AND LATITUDE, ate. It remains then to treat of time relative values of the various means of estimating a ship's true position by Means of certain operations called " Failings." In considering first the nature of the most simple form of naviga tion, namely, plane sailing, but which is only available for small spaces on the earth's surface and in low latitudes, the following dia gram will aeeist, Suppose for example, a ship bound to a port beaming true N.W., distance 80 miles or knots. Her average general velocity to be knots per hour. A W.N.W current to be setting at the rate of 3 miles per hour. The terrestrial deviation on the compass to be 2 points westerly. The ship's local attraction upon the average course to be 16° westerly : while, moreover, the wind being north-easterly (true), and the ship ,being nearly close-hauled, we should allow about I point leeway. It is obvious that with so many elements of disturbance an approximate course must be selected before starting. Now, the un scientific ship-maeter would guess at his course, and if he found himself by subsequent observation or otherwise, setting either to leeward or the reverse, would so vary his course as to attempt by time-losing ex periments to finally reach his destination ; while the complete navigator would probably proceed thus :—He would either roughly calculate his true course, having in his mind's eye the following figure, or would construct the figure itself, as under :— herself at R. By middle latitude sailing the mean between the paralle left and that reached would be 44°+ 46* = 45°, but if we measure 2 along the track sailed, it will be found that one-half the distance sailed would actually fall to the southward of 45°, or at C; hence the inaccuracy of middle latitude sailing in finding the longitude. Of
course the remedy for this would be to divide the track into portions, and find the difference of longitude for each. When near the equator where the meridians are not so convergent, middle latitude sailing may be used with very trifling error ; but Mercator's sailing is at such places less accurate, because, as the following figure will show, a small error in the course would make a large error in longitude.
Let o represent the ship's position at starting, and A the intended port. It is usual to take the current as a course and distance. We get the approximate distance by saying in this case 8 : 80 : : 3 : 30. Os will, therefore, represent in position and magnitude the current course and distance ; compounding the ship's' bearing and distance from her intended port, and the set of current, as forces [CoarrosrrioNi, we get the parallelogram o z A y, and the L A o y (composed with the N.W. line Os) will be the course, which measured from north would be about N. 324' W., or by calculation, thus : o A being given as 80, and bearing N.W., and o z being given 30, bearing W.N.W., the 0 A would = 2 points : hence in the triangle A o y we have given the two aides and included angle o A y A) to find' the rest by trigonometry. From this it would be seen that starting from o, in the direction o y, would, as influenced by the current, carry the ship along the resultant o A to its intended port at A, were no other in fluence at work. But perhaps the most difficult portion of a mariner's duty is to clear his course from compass errors. To continue the example;—after projecting the 2 points variation westerly from n to c, and the 16' W. by local attraction [Locum. ArrnAcTros] from c to n, and the 1 point leeway (westerly) from n to E, we find that the correction for leeway and compass error amounts in this case to the correction n E, which, measured to the eastward of o if, gives the true course which must be steered by compass to be o r, or instead of about N. 324' W., it should be about N. IS' E.