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134.—The Magnetic Variation of Declination in Time, becomes important in reference to old plans in which the magnetic north of the period has been plotted for the true north very much to the pecuniary advantage of the legal profession when engaged upon actions with regard to disputed boundaries. The following table gives an idea of the variation in declination for Greenwich approximately for a few dates:—
| Year | 1580, | Dec. | 11° 36′ E. | Year | 1860, | Dec. | 20° 40′ W. |
| " | 1663, | " | 0 | " | 1870, | " | 20° 19′ W. |
| " | 1700, | " | 8° 20′ W. | " | 1880, | " | 18° 58′ W. |
| " | 1818, | " | 25° 41′ W. | " | 1890, | " | 17° 9′ W. |
| " | 1850, | " | 19° 31′ W. | " | 1900, | " | 16° 30′ W. |
It will be seen by the above table that the needle pointed due north in 1663, that it attained its greatest western declination in 1818, and that it is now losing its westerly declination at the rate of about 7′ annually.
135.—Annual Variation.—The declination is subject also to a small annual variation which is greatest about spring time, diminishes towards the summer solstice, and increases again during the following nine months. It varies at different periods, and seldom exceeds 16′ of arc.
136.—Declination Correction to true north may be made for the compass by observation in this hemisphere of the pole star, which is practically due north in January at 6 p.m., February at 4 a.m., March at 2 a.m., April at midnight, May at 10 p.m., August at 4 a.m., September at 2 a.m., October at midnight, November at 10 p.m., December at 8 p.m. Most surveying instruments, except the transit theodolite, are not made convenient for this observation. More generally observations of the position of the sun may be made where a sun-glass is provided to the telescope of the theodolite, [Fig. 19], SG, page 45, with the aid of a chronometer or a good watch. For this observation we may remember that the sun is true south at twelve o'clock on the 16th April, 15th June, 1st September, and 25th December. The following table may be useful for some intermediate times to show how much the chronometer (mean time) is faster or slower than the sun's southing approximately at noon:—
| Jan. | 1 | subtract | 4 | min. | July | 15 | subtract | 6 | min. |
| " | 16 | " | 10 | " | " | 30 | " | 6 | " |
| " | 31 | " | 14 | " | Aug. | 14 | " | 4 | " |
| Feb. | 15 | " | 14 | " | Sept. | 13 | add | 4 | " |
| Mar. | 2 | " | 12 | " | " | 28 | " | 9 | " |
| " | 17 | " | 8 | " | Oct. | 13 | " | 14 | " |
| April | 1 | " | 4 | " | " | 28 | " | 16 | " |
| May | 1 | add | 3 | " | Nov. | 12 | " | 16 | " |
| " | 16 | " | 4 | " | " | 27 | " | 12 | " |
| " | 31 | " | 3 | " | Dec. | 12 | " | 6 | " |
| June | 30 | subtract | 3 | " | " | 31 | subtract | 3 | " |
137.—As variation in time of southing is from fourteen minutes fast to sixteen slow, or a difference of thirty minutes, correction becomes important, as the sun passes over 7½° in this period. In these observations the diaphragm lines, webs, or points must bisect the sun's disc. This is done more exactly by taking the mean positions of the sun's eastern and western limbs or its semi-diameter, which is given for every day of the year in the Nautical Almanac.
138.—The Compass-box.—The needle, as it is generally mounted for the theodolite, mining-dial, and many other instruments, reads into a divided circle of 360°. The circle is raised up from the bottom of the compass-box to the height of the top of the needle, as shown in section [Fig. 30], D, and is generally silver-plated. The bottom of the compass-box is sometimes divided with a compass-rose giving the points N. E. S. W. The E. and W. in some cases are reversed from their natural directive positions from the centre of the box, so as to read the letter indicating the point nearest to the division instead of that opposite to it. In modern surveying instruments, however, no regard is paid to the points of the compass, north being 0°, east 90°, south 180°, west 270°.
139.—In the manufacture of the compass-box very great care should be taken that the metal is quite free from iron, and that no iron comes near it. On this point the maker cannot be too guarded. The author has in several instances found the compass-box of perfectly free metal; but a single foul screw made of commercial brass wire, being used to fix the ring or the rose plate, has by its influence entirely destroyed the value of the compass.