RULE LXXII.

1o. With ship time and longitude in time find the Greenwich date Rule LV, page 144).

2°. Take from page II, Nautical Almanac, the sun's declination and reduce it to Greenwich date (Rule LVI, page 147; also take out sun's semidiameter.

3. Correct observed altitude for index error, dip, refraction, parallax, and semi-diameter, and thus get the true altitude (Rule LX, page 159). 4. Proceed according to Rule LXX, page 211, to find the true azimuth. 5°. Having found the true azimuth, proceed by Rule LXXI to find the

variation.

EXAMPLES.

Ex. 1. 1872, May 19th, 3h 7m 44 P.M. mean time at ship, latitude 41° 53' N., longitude 60° 19′ W., sun's bearing by compass S. 94° 40′ W., observed altitude sun's L.L. 43° 56′ 7′′, height of eye 18 feet, index corr. o': required the true azimuth and variation.

Ex. 2. 1872, September 2nd, mean time at ship 8h 59m A M., latitude 39° 31′ S., longitude 127° 45′ W., sun's bearing by compass N. 39° 34' E., observed altitude sun's L.L. 26° 40' 37", height of eye 18 feet: required the true azimuth and variation.

Ex. 3. 1872, July 5th, mean time at ship 6h 55m 519 P.M., latitude 50° 53' N., longitude 119° 8′ E., sun's bearing by compass N. 65° 30′ W., observed altitude sun's L.L. 9° 40′, index correction + 3′ 50′′, height of eye 18 feet.

Ex. 4. 1872, February 10th, at 8h 2m A.M, mean time at ship, latitude 50° 48′ N., longitude 77° 30' W., sun's bearing by compass E. by S. & S., observed altitude sun's 1' 6", height of eye 15 feet: required the true

L.L. 7° 10' 40", index correction

azimuth and variation.

Ex. 5. 1872, January 21st, at 10h 14m A.M. app. time at ship, latitude 39° 3′ S., longitude 96° 28' E,, sun's bearing by compass E. 2° 30' S., observed altitude sun's U.L. 46° 15', index correction 2' 43", height of eye 19 feet: required the true

azimuth and variation.

Ex. 6. 1872, June 1st, at 9h 40m A.M. mean time at ship, latitude 60° N., longitude 40° 20′ W., observed altitude sun's L.L. 44° 48′ 50′′, index correction + 3' 17", height of eye 18 feet, sun's bearing by compass S. variation.

W.: required the true azimuth and

True altitude (Norie) 45° 3′ 0′′, hourly decl. June 1st at noon, viz., 22° 8′ 25′′ N.,

The Greenwich date is June id oh 21m 208. diff. of decl. 19" 87 × 3h=7′′, which, added to gives the red. decl. 22° 8' 32" N., polar distance 67° 51′ 28", sum of logs. 19:220454, true azimuth S. 48° 6' E.

True azimuth S. 48° 6' E.

S.point W. Mag. azimuth S. 5 37 W.

Variation 53 431 W.

The true azimuth being to the left of the magnetic.

EXAMPLES FOR PRACTICE.

In each of the following examples it is required to find the true azimuth and variation :

No. Civil date. 1872. 1. Jan. 24th, 2. Feb. 28th, 3. Mar. 27th, 4. April 3rd, 5. May 27th, 6. June 20th, 7. July 31st 8. Aug. 23rd, 9. Sept. 1st, 10. Nov. 25th, 11. Dec. 17th, 12. July 3rd, 13. Jan. 6th, 14. April 25th, 15. Jan, 29th, 16. Feb. 1st, 17. March 26th,

M.T. ship.

8b2235 A.M.

3 14 O P.M. 4 6 40 P.M. 620 O P.M. 9 3 20 A.M. 610 O P.M. 8 46 30 A.M. 5 54 58 A.M. 3 47 50 P.M. 4 7 O P.M. 9 10 30 A.M. 8 26 50 A.M. 5 2 14 P.M. 7 56 41 A.M. 3 36 35 P.M. 3 44 51 P.M. 9 5 50 A.M. 0 P.M.

18. Feb. 26th,

2 48

5

o N.

167

o E. N.

19. June 21st,

322

O P.M.

66 40 N.

55 20 W. S.

o W. N.

0 25 E.

21. 1873, Jan. 1st, 9 27 10 A.M.

ON FINDING THE LATITUDE BY REDUCTION TO THE MERIDIAN.

THE latitude of a place is most simply determined by observation of the meridian altitude of a known heavenly body. When such an observation cannot be obtained by reason of the state of the weather, the altitude of the body may often be obtained a little before or a little after its meridian passage. And if at the time of observing such an altitude near the meridian, the hour-angle of the body is known, we may find by computation very nearly the difference of altitude by which to reduce the observed to the meridian altitude. The correction is called the "Reduction to the Meridian." This method, in point of simplicity, is little inferior to the meridian altitude, to which it is next in importance. The latitude may also be determined by a direct process, deduced from spherical trigonometry. The former is the method used in the following pages. The term 66 near the meridian" implies a meridian distance limited according to the latitude and declination, and also the degree of precision with which the time is known (see Raper, Table 47).