tion, according to whether these have the same or different names.

9. A compass Bearing, if it is correct magnetic, requires correction only for Variation; otherwise it must be corrected for variation, and also for the Deviation due to the direction of the ship's head or course; in both cases, Easterly to the right, and Westerly to the left; the result is the true Searing.

10. When a true course has been taken from the chart, and has to be converted into a compass course, apply

Easterly Variation to the left, but Westerly Variation to the right.

So also, with regard to Deviation, apply—

Easterly Deviation to the left, but Westerly Deviation to the right.

If there is no Deviation, the true course, by the application of the Variation, becomes the compass course.

But if there is Deviation, the application of the Variation merely converts the true into a correct magnetic course; and to the latter must be applied the Deviation due to that particular correct magnetic direction of the ship's head; which being done the result is the compass

PART II.

ON FINDING THE LATITUDE AND LONGITUDE BY

OBSERVATION.

Although the sextant is a superior instrument to the quadrant, the latter will answer our purpose sufficiently well, if care be taken to purchase a good one from a respectable maker.

A quadrant is a reflecting instrument, in which the arc is divided a few degrees beyond 90°; in the sextant the graduations extend a few degrees beyond 120°: the intermediated graduations in the latter instrument are also more minute than those in the former.

For a full explanation of the sextant, the reader is referred to the "Treatises on Mathematical Instruments," Nos. 32 and 170, forming volumes of this series of Rudimentary Works.

To read off the quadrant.—It will be seen that the arc of the instrument is divided into degrees, marked 18, 19, 20, &c. Each of these is again divided into thirds, each therefore representing 20'.

The vernier scale is that which is movable. As the arc is divided into degrees and minutes, so is the vernier scale divided into minutes and seconds only. The nonius on it is marked thus

To ascertain

the amount of degrees, minutes, and seconds denoted by the instrument, observe what line of the vernier

scale coincides with a line on the arc of the instrument. The engraved arrow is in this instance inserted to point out to the learner the coinciding lines. The nonius is somewhere between 23° and 24°; that is, it points out 23° 20' at the least: for the excess or remainder we must now look at the vernier scale below. Here the coinciding line is the second division between 7' and 8'. Here, then, 7' 40" are to be added to the

ADJUSTMENTS OF THE QUADRANT AND SEXTANT.

1. The first adjustment is to set the index glass perpendicular to the plane of the instrument.

To make that adjustment, place the vernier near the middle of the arc and look obliquely into the index glass to see if the arc and its image are continuous; if they are not, slacken or tighten (as required) the proper screws.

2. The second adjustment is to set the horizon glass perpendicular to the plane of the instrument.

Set 0 on the vernier to 0 on the arc; then hold the instrument obliquely, and look from the sight vane through the horizon glass to the horizon; if the horizon and its image form one line, the glass is perpendicular; if not, make it so by the screw for that

purpose.

3. The third adjustment is to set the horizon glass parallel to the index glass, with 0 on the vernier set to 0 on the arc.

To make the third adjustment, hold the instrument vertically, and look from the sight vane through the horizon glass to the horizon: if the horizon and its image form one line the horizon glass is parallel to the index glass; if not, make it so by the screw for that purpose.

4. In the absence of a screw or screws, find the index error by the horizon, as follows:—Set 0 on the vernier to 0 on the arc, hold the instrument vertically, and look at the horizon through the horizon glass; if the horizon and its image are not in one line, move the tangent screw till they are so; the reading is the error, which is to be added if the reading is off the arc, to be subtracted if on the arc.

5. Or you may find the index error by the sun as follows:—Turn on the shades adapted to your sight, hold the instrument vertically, the index being near 30' on the arc; then look at the sun and move the index till the edges of the sun and its image just touch; take this reading, which is on the arc. Then place the index near 30' off the arc, and again make the edges of the sun and its image touch; take this reading, which is off the arc. Half the difference of the two readings is the index error, to be added if the greater reading is off the arc, to be subtracted if the greater reading is on

TC.

Ex. Reading off the arc 33', reading on the arc 29'; half the difference is 2' to be added.

To know whether those measurements or angles have been taken with tolerable accuracy, the sum of the two readings, divided by 4, should be the sun's semidiameter for the day as given in the Nautical Almanac.

Our next step must be to know how to convert longitude into time, and vice versa, both which operations are constantly in use in finding latitude and longitude.

RELATION OF DEGREES (or ARC) TO TIME.

The earth rotates on its axis once in 24 hours, and its circumference is 360 degrees; hence, if we divide the degrees by the hours (360 by 24) we have the following relation of arc (or longitude) to time, viz. 15° for lb., 15' for 1m., and 15" for 1 sec., whence-

1'

1"

1° will correspond to 4 m. (i.e. the 15th part of 1 hour or 60 m.). 4 s. (i.e. the 15th part of 1 minute or 60 s.). 4 thirds (i.e. one-sixtieth of a second).

and the following rules are sufficiently clear.

To convert Degrees, &c., of Longitude into Time.

Rule.-Multiply by 4; then the degrees of Long. become minutes of time; the minutes (') of Long. become seconds of time; and the seconds (") of Long. become thirds of time.

Similarly 18° 54'1h. 15m. 36s.; and 100° 14'6h. 40m, 56s.