Care should be taken to examine the compasses, in order to ascertain whether they are not affected by something on board the ship. When the compass is in use be careful that the lubber's point be placed perpendicular to the fore side of the binnacle, and when lying in port and the compasses not in use, the compass card should be taken off the spindle point and wrapped together in pairs in dry brown paper, with the north point of one to the south point of the other, and kept at a distance from other pairs, which will improve their magnetic virtue. From the above considerations you will perceive how important a matter it is to be enabled to detect and rectify the above errors and have a thorough understanding of navigation. LECTURE No. 8. Gentlemen: the importance of a thorough understanding of the finer branches of navigation, such as working the moon and planets, can only be appreciated by those who have been obliged to make a European passage without seeing the sun for weeks; and the object of this lecture will be to explain the best method of obtaining the latitude of the ship by meridian altitude of the moon, and a simple rule for the same. In the first place it is necessary to obtain the meridian passage of the moon for the place of the ship, which is done by entering the Nautical Almanac, and find the moon's passage for that day at Greenwich and her passing the meridian the next day, and the difference between these two times, which is found at the top of table 28, under which and opposite the longitude of the ship will stand a correction to be applied to the time of passage at Greenwich to obtain her passing the meridian of the ship. Having obtained the hour of her passing the ship, I obtain her altitude by the sextant, augment the semi-diameter from Nautical Almanac, and apply it to the altitude, subtract the dip to obtain the apparent altititude, find a correction for parallax and refraction, by means of table 19; this correction must in all cases be subtracted from the standing correction 59′ 42′′, the differences of which must be added to the apparent altitude, to obtain the true altitude. This true altitude subtracted from the angle 90° will give the zenith distance; correct the declination in the usual manner and apply it to the zenith distance; if of the same name, add them; if of different names, subtract them, and the result will be the latitude of the ship at the hour of observation. This problem is of great importance to the navigator, a thorough understanding of which will enable him to obtain his latitude at night, and by using it in Sumner's method he may also obtain his longitude and his bearings and distance off the land. LECTURE No. 9. In this lecture I shall explain a very important problemhow to obtain the longitude of the ship by the moon. In the present improved state of the naval Nautical Almanac we can easily find the moon's right of ascension and declination for the time at Greenwich, and even without a chronometer the time at Greenwich can be obtained, if we know the longitude of the ship, as well as the mean time at the place of observation, by a watch; for by adding the longitude in time to the time by watch, if in west longitude, or subtracting it if the longitude is east, we shall obtain the corresponding time at Greenwich. Great accuracy is required in an observation of this kind in computing the time at Greenwich, for an error in this will affect the moon's right ascension and declination, which vary rapidly. Right ascension often 3" in one minute of time, and declination 15" in one minute of time. In taking an observation note the time by a chronometer; at the same instant take from the Nautical Almanac the sun's right ascension, moon's right ascension, moon's declination, moon's horizontal parallax, moon's semi-diameter, which must be augmentated and added to it. To the observed altitude apply the moon's semi-diameter and subtract the dip; to obtain the central altitude, add the correction for parallax and refraction (which correction is found in table 19) by subtracting the tabular number corresponding to the altitude and horizontal parallax from the standing number of 59′ 42′′, the correction so found to be in all cases added to the central altitude to obtain the correct altitude, then bring forward the latitude to the time of observation. Obtain the polar distance in the usual manner, add together the corrected altitude, latitude and polar distance to obtain their sum, halve this sum, and from this half sum subtract the moon's true altitude for a remainder, get the secant of the latitude, co-secant of the polar distance, cosine of the half sum, and sine of the remainder, half the sum of these four logarithms will correspond to time in the hour angle column, apply the moon's right ascension, the sum or difference will be the right ascension of the meridian; from this subtract the sun's right ascension (increased by twenty-four hours if necessary), the remainder will be the apparent time of the ship; to this apply the equation of time, and it will give the mean time of ship at the place; and the difference between this mean solar time and the time shown by the chronometer at the time of observation will be the longitude in time, which is turned into degrees, minutes and seconds by table 21, and will be the longitude of the ship at the place of observation. Sumner's method may also be worked by the moon, and the navigator be enabled to ascertain his true position at any time of night. ᎬᎡᎡᎪᎢᎪ . Example 1st, page 40, for 29 nautical miles, read 20.1 miles. Example 2d, page 40, for 31 nautical miles, read 22.5 miles. Velocity of sound, page 42, for 83 nautical miles, read 6.3 miles. Page 43. To find the Index Error, for one half the difference of the two readings, read, the difference of the two readings will be the Index Error. Page 47, question 4th, for multiply, read divide. Page 84. For sometimes addition and sometimes subtraction, read sometimes added and sometimes subtracted. Page 46. Days Work. In remarks, read, latitude left yesterday noon was 13 degrees, 25 mins., N. Longitude 30 degrees, 15 mins., W. A current setting 1 mile per hour, E. § N. Variation of the compass, 1 points W. |