In Fig. 20, since the true azimuth O S' falls to the left of the compass azimuth O B, the total error of the compass is westerly; and since the compass north falls to the right of the magnetic north, the resulting deviation is easterly.

Comp.N

Mag.N

S

N

FIG. 20

19h 11m A. M.

E

8h 40m

10h 31m A. M.
16m 16s

10h 47m 16s A. M.

Sun's Decl., Nov. 6, 1899 = S 16° 0′ 46′′

In this case, the latitude and the declination have different names. Therefore, enter the third section of the Azimuth Tables and find there, for latitude 47°, declination 16°, and 10h 50m A. M. apparent time, the true azimuth, N 161° 23′ E. Ans.

EXAMPLE 3.-On July 7, 1899, about 4 o'clock in the afternoon, when the chronometer indicated 10h 24m 34s, a bearing of the sun was taken. Latitude in = 27° N. Longitude in 92° 30′ W. Find the true azimuth.

Sun's Decl., July 7, 1899 = N 22° 35'.

In this case, the value of the declination is nearly 22°. Therefore, in picking out the azimuth from the tables, take the mean of the

azimuths given for declinations of 22° and 23°, respectively. Thus, for latitude 27°, apparent time 4b 10m P. M., and 22° of declination, an azimuth of 80° 23' is obtained; and with the same latitude, apparent time, and 23° of declination, the corresponding azimuth is 79° 14'. Hence, the true azimuth is

80° 23′+ 79° 14′

159° 37'

= N 79° 48′ W. Ans.

2

EXAMPLES FOR PRACTICE

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1. On February 7, 1899, at 5h 48m A. M., local apparent time, the sun's bearing by compass at rising was east. Heading of ship, NN W. Latitude in = 10° 20′ S. Longitude in 1° 30′ W. Find the amplitude, the total error of the compass, and the deviation for heading, the variation according to chart being 22° 20′ W.

=

2. On November 25, 1899, at 8h 11m 12s P. M., local apparent time, when steering W by S, the sun's bearing by compass, at setting, was WN. Latitude in 55° S. Longitude in 122° 45′ E. Required, the total error of the compass and the deviation for heading, the variation by chart being 10° 1′ W. Total error = 41° 1' W Dev. for W by S = 31° W

Ans.

3. In the morning of February 3, 1899, the observed altitude of the sun's lower limb was 16° 2′ 15′′. Compass bearing at time of sight was SE by EE, the ship heading E by N. The chronometer, which was 2m 15s fast on Greenwich mean time indicated exactly at that instant, 9h 30m 45s. At the time of observation, the position, by dead reckoning, was 47° 15′ N and 179° W. Height of eye 23 feet. Index error = +3′ 15′′. Assuming the variation to be 24° E, what was (a) the sun's true azimuth? (b) the deviation of compass for this heading? (a) True azimuth = S 41° 38′ E 1(b) Dev. for E by N = 6° 34' W

4.

Ans. {

=

On March 20, 1899, in latitude 56° 20' S and longitude 63° 42′ W, at 4h 4m P. M., local mean time, when heading east, the observed altitude of the sun's upper limb was 16° 54′ 30′′. Index error = +1' 1". Height of eye = 27 feet. At the instant of measuring the altitude, the sun's bearing by compass was W S. Find the total error and the deviation for heading, the variation by chart being 24° 17' E.

5. On September 1, 1899, in the forenoon, the observed altitude of the sun's upper limb was 34° 28' 10". Index error = -2′25′′. Height

of eye 23 feet. Sun's bearing, by compass, at instant of observation = SEE. Latitude in = 44° 18′ N. Longitude in = 10° 15′ W. Course steered, WS W. Greenwich mean time at observation, August 31, 21h 22m 37s. Find the total error of the compass and the deviation, assuming the variation to be 12° W.

6. On July 2, 1899, at about 2h 45m P. M., the latitude and the longitude of a vessel were respectively 45° N and 51° 10′ W. The navigating officer deemed it advisable to test the accuracy of the deviation table for the four intercardinal points, and accordingly the compass bearing of the sun was taken when the ship was heading on these points successively and was found to be as follows:

The mean value of variation as taken from the chart is 30.2° W. Find by means of the Azimuth Table (see specimen page) the correct deviation of compass for these quadrantal points.

7. On November 13, 1899, in the morning, in latitude 45° S and longitude 128° 30′ W, the sun's compass bearing, as found with an azimuth instrument, was N 61° 30′ E. At the instant of observation, the Greenwich mean time by chronometer was November 13, 5h 18m 44 P. M. The ship was heading NNE. The variation of the locality, by chart, was found to be 17.5° E. Find by computation and also by the Azimuth Table (see specimen page): (a) the sun's true azimuth; (b) the deviation corresponding to this direction of the ship's head. JTrue azimuth, by computation S 110° 56′ E True azimuth, by table method S 110° 55′ E Dev. for N N E, by computation Dev. for N N E, by table method

Ans.

(a)

(b)

=

9° 56' W = 9° 55' W

AZIMUTH INSTRUMENTS

44. The best way of observing the compass bearing in amplitude and azimuth observations is to use what is known as an azimuth instrument. Different types of such instruments are now in use on shipboard, but the principles on which they are constructed are about the same.

These

FIG. 21

instruments are very useful, because they afford a quick and accurate estimate of the bearing, and a convenient means of checking, at any time, the accuracy of deviation determined by the process of "swinging in port." For this reason, azimuth instruments should be included in the navigating equipment of every iron and steel vessel.

45. In Fig. 21 is shown an azimuth instrument attached to the compass and ready for use. The principal features of this instrument, which is known as the Ritchie bar azimuth instrument, are that it can be used for both star and sun observations and for taking the bearing of terrestrial objects. This instrument is provided with a black-glass reflector a, a mirror b, and a prism c, and is swung from a center post that enters a socket, or indentation, drilled in the center of the compass glass. When observing the bearing of the sun, the mirror b is inclined so that a ray of the sun (represented by a solid line) is reflected from 6 to the prism c. This ray enters the case holding the prism through a vertical slit shown in the figure, whence it is diverted downwards, appearing as a bright bar of light on the graduated card of the compass.

When observing a star, its light is reflected from a (see dotted line), in line with the vertical thread h on the sight vane, to the observer's eye, whence the bearing of the star is read through the slit d at the intersection of the horizontal hair line with the graduation on the card below. When taking bearings of terrestrial objects, the observer brings the object in line with. d and h and reads off the bearing as before, being guided by the horizontal hair line.

46. Another type, called the Bliss azi

muth instrument,

FIG. 22

is shown in Fig. 22. Like the instrument just described, it is fitted with two sight vanes a and b that are exactly opposite each other and in line with the center of the card when attached to the glass cover of the compass bowl.