Ex. 4. 1869, November 10th: find A.M. and P.M. tides at Portland Breakwater. In this case the standard port of reference is Portsmouth, and the first tide at Portsmouth occurs at 332 A.M. (i.e., 3h 32 after midnight), consequently, since Portland constant shows that the tide there occurs 4 40 earlier than at Portsmouth, and that quantity subtracted from November 104 3h 32m A.M. would give a P.M. tide of the 9th at Portland; we therefore use the Portsmouth tide of the 10th, P.M., and of the 11th, A.M., thus: Ex. 5. 1869, November 28th: find A.M. and P.M. tides at Falmouth. The standard port of reference is Devonport. A blank (-) occurs in the morning column of the 28th, we therefore use the next tide (as the constant is subtractive), viz., the P.M. tide, 12h being added to enable us to make the subtraction, thus: Time H.W. Devonport, Nov. 28th oh 27m P.M. (next tide) 29th, 1h 9m A.M. Constant -0 46 O 23 A.M. Times H.W. Falmouth, Nov. 28th 11 41 A.M. Hence, there is no P.M. tide on the 28th at Falmouth. Ex. 6. 1869, February 25th: find the A.M. and P.M. tides at Flushing. Ex. 7. 1869, April 5th: find A.M. and P.M. tides at Milford Haven (entrance). Time H.W. Pembroke, 5th oh 2m A.M. Constant No. A.M. tide. 5th, oh 41 P.M. 20 5th, o 21 P.M. Ex. 8. 1869, May 13th: find A.M. and P.M. tides at Ramsey. Time H.W. Holyhead, 13th, 10h 58m A.M. Constant + I I Time H.W. Ramsey, 13th, II 59 A.M. No P.M. tide at Ramsey on the 13th. 13th, 11h 15m P.M. 12 16 12 O 14th, o 16 A.M. BB EXAMPLES FOR PRACTICE. In each of the following examples it is required to find the times of high water, A.M. and P.M. : IN pages 149 to 216, of Admiralty Tide Tables for 1869, are given the times of high water at full and change, by which we are enabled to calculate approximately the time of high water on each day. The constant is found by taking Brest as the standard port, at which place the time of high water, full and change, is 3h 47m. The difference between the full and change at the given port and Brest will be the constant, to be employed as in the preceding Rules, except there be a great difference of longitude; in which case the correction for the moon's meridian passage must be employed, since for the greatest longitude this correction may amount to half an hour. Should the longitude, however, not exceed 5°, it may be neglected, as doing so will scarcely make more than a difference of one minute. It must also be observed that the longitude of Brest is 44° W. of Greenwich, and in strictness, therefore, in determining this correction 4° should be subtracted, if the longitude of the place be east, or added if it be west. The correction is found in Table XVI, Norie, or Table XXVIII, Raper. EXAMPLES. Ex. 1. March 5th: required the times of high water at Victoria River (N.W. coast of Australia), longitude 130° E. Time of H.W. full and change, Victoria River Brest....... 7h15m (p. 214) Constant+3_28 4 E. 126 Under 52 and against 126° longitude, in Raper, Table 28, or Norie, 16, we find 18m to be subtracted, because the longitude is E. Ex. 2. 1869, October 11th: find the times of high water at Sandy Hook, longitude 74° W. Time of H.W. full and change, Sandy Hook.... 7h 29m (p. 209) Brest..... 3 47 (p. 184) Under 53 and against 78° longitude, in Raper, Table 28, or Norie, 16, we find 11m to be added, because the longitude is West. Ex. 3. 1869, May 10th: required the times of high water at Nelson, New Zealand, longitude 173° E. Time of H.W. full and change, at Nelson..... 9h 50m (p. 202) Constant +6 3 D's transit, 10th, 11h 7m A.M. 9th, 10 23 44 Under 44 and opposite 169° (173° — 4°) in Table 16, Norie, or 28, Raper, stands the correction 20 to be subtracted. Ex. 4. 1869, August 9th: find the times of high water at Cape Virgin, Straits of Magellan, longitude 68° W. Time of H.W. full and change, Cape Virgin.... 8h 30m (p. 214) Brest... 3 47 (p. 184) Constant + 4 43 55m and long. 72° W. (68° + 4°) give corr. + 10m. Ex. 1. 1869, July 18th: find the times of high water at Caracus River, Ecuador, longitude 67° W. Ex. 2. 1869, September 20th: find the times of high water at Auckland, New Zealand, longitude 175° E. Ex. 3. 1869, May 14th: find the times of high water at Point de Galle, Ceylon, longitude 80° E. Ex. 4. 1869, February 27th: find the times of high water at San Francisco Bay, longitude 122° W. GREENWICH DATE BY CHRONOMETER. THE ERROR OF CHRONOMETER ON MEAN TIME at any place is the difference between the time indicated by the chronometer and the mean time at that place. The error of chronometer on Mean Time at Greenwich is the difference between the time indicated by the chronometer and the mean time at Greenwich. The error is said to be fast or slow as the chronometer is in advance of or behind the mean time at Greenwich. RATE OF CHRONOMETER is the daily change in its error, or the interval it shows more or less than twenty-four hours in a mean solar day. If the instrument is going too fast, the rate is called gaining; if too slow, losing. When the error of a chronometer on Greenwich mean time, and also its daily rate, are known, we may determine Greenwich mean time at some other instant, as when an observation is taken, by the following: RULE LXVI. 1°. To the time by chronometer apply the original error, adding it if the chronometer was slow, rejecting 24h if greater than 24", and putting the day one forward; but if chronometer is fast, subtract original error, increasing time shown by chronometer by 24 if necessary, and putting the day one back. 2°. Multiply the rate of chronometer by the number of days elapsed since the original error was ascertained, and add thereto the proportionate part for the fraction of a day, found by proportion or otherwise; the result is the accumulated rate in the interval. 3. To the result found by 1°, add the accumulated rate, if chronometer is losing; but subtract if gaining; the result will be mean time at Greenwich, at the instant of observation. EXAMPLES. Ex. 1. 1872, January 30th, P.M. at ship, time by chronometer 29d 15h 47m 48.3, which was 10m 248.7 slow on Greenwich mean time Jan. 1st, and losing 21 daily: required Greenwich date by chronometer. Acc. rate imos. I Ex. 2. 1872, March 20th, P.M. at ship, an observation was made when the time by chronometer was 20d oh 7m 55, which was 50m 51 fast on Greenwich mean time Nov. 22nd, 1871, and losing 68.8 daily. Time by chron., March 20d oh 7m 55 Nov. 30 Rate 6.8 118 22 |