We have made these remarks with a view to impress on Navigators, the importance of making themselves perfectly acquainted with Time, which can be done by a careful perusal of the article on that subject in their "Epitome," and it cannot be too strongly urged, that it is also especially desirable, that every one using the Nautical Almanack, should make himself thoroughly acquainted with its contents, to accomplish which, it is necessary to peruse very attentively, the Explanations," at the end of that book. TIDES. ON FINDING THE TIME OF HIGH WATER. THE mean time of High Water at London Bridge is given in the Nautical Almanack, for every day of the year, on the assumption that the time of High Water on full and change days, or the Establishment of the Port, is 2h 7m. The first High Tide which happens after mean noon of any day is inserted in the first column, and the second in the second column. Where a line (—) is inserted, it indicates that there is only one High Tide on that day, thus, on January 20th, there is only one High Tide; it occurs at 11h 29m, but the succeeding High Tide does not take place until 1m after mean noon of January 21st. astronomical time. The times of High Water at full and change of the moon as given at pages 492 and 493 of the Nautical Almanack for 1868, are reckoned from apparent noon; they represent the Establishment of the Ports, that is, the actual times of High Water, when the moon passes the meridian at the same time as the sun. They serve to determine the time of High Water, on any other day, at those places in the following manner: Rule. From the Nautical Almanack, pages 492 and 493, take out the Establishment of the Port or the times of High Water on full and change at London Bridge and the given place. Find the difference and make it when the High Water at the given place is more than that at London Bridge, Then take the times of High Water at London Bridge for the given day, from page 490, of the Nautical Almanack, as follows: The morning tide is to be taken from the second column and opposite the preceding day, and must be dininished by 12m. The afternoon tide, from the first column and opposite the given day of the month. Should a blank fall in either column, take the tide preceding, when the Establishment of the Port is greater than that at London Bridge, but take the tide following the blank when it is less. To these times apply the difference or CONSTANT, adding or subtracting it according as it is marked + or — The results will be the A.M. and P.M. tides respectively, P.M. if the result be less than 12h, but A.M. on the From this subtract the 12h, and the remainder will be the As the Admiralty Tide Tables are not published in advance of the current year, the questions on tides in the papers are calculated from the foregoing rules in the Nautical Almanack. THE ADMIRALTY TIDE TABLES Give the A.M. and P.M. times of High Water, and the height of the tide for every day in the year, at the following places which are called STANDARD Ports. Hull, Weston-super-mare, Waterford, The method is so simple that it is scarcely necessary even to give an example. Required the A.M. and P.M. tides at Berwick, January 6th, 1865. Time of High Water at Sunderland, the standard port, Constant for Berwick 10h 5m a. m 9 1 a.m. at Berwick. 10h 40 p.m. -1 4 9 36 p.m. These Tables contain so much useful information respecting the tides, the different set of the currents surrounding the British Isles, &c. &c., that the pupil should procure a copy of the work. The time of High Water is given to mean time at the place; the apparent time can be obtained by applying the equation of time by addition or subtraction as directed for that purpose. The height of the tide in these Tables is calculated from the mean level of the low water of ordinary springs, because the soundings in most charts are reduced to that level. The height therefore which is given at each place, is the actual rise of High Water above the mean Low Water level of spring tides. The following Rule will also be found useful to find what water there may be on any Bar or Harbour at any time of tide. Admiralty Tide Tables. To find what Water there may be on any Bar or Harbour, at any time of tide. t Rule. To the difference between the height of high water at the given place, and the half mean spring range, add to, or subtract from it the number found in Table B in the Admiralty Tide Tables, page 98, according to the direction there given; the result will give the height of the tide above zero, or low water mark, at the time required. Examples.-Required the height of the tide above zero, at Liverpool, on March 19, A.M. at 4h after high water. The general method of finding High Water, and that which is used at some of the out ports in the Examinations, will be found fully explained with Examples in the Epitome. G ORDINARY EXAMINATION. In working the Questions in these Papers, the general Tables used must be taken from the ‘Epitome,' but the Declination, Equation, Semi-diameter, &c., will be found at the end of the Hand Book, as well as the p. logs for correcting the O's declination. PAPER 1. For Second Mate. MULTIPLICATION BY LOGARITHMS, Rule. Add together the logarithms of the factors, and the natural number answering to the sum, will be the product sought. If either or both the factors should consist wholly of decimals, and the index of the sum exceed 10, reject 10, and the remainder will be the index of the logarithm answering to the product. 1. Multiply 76 by 43 by logarithms. The log. of 76 is 1.880814 43 1.633468 Rule. From the logarithm of the dividend subtract the logarithm of the divisor, and the remainder will be the logarithm of the quotient. If the logarithm of the divisor be greater than that of the dividend, increase the index of the latter by 10. 2. Divide 144 by 4 by logarithms. 3. In latitude 33°, the departure made was 130 miles : required the difference of longitude by parallel sailing. |