cannot be affected by the deviation; for the attractions of the point p and the earth in this case coincide: but when the head of the vessel is directed towards the magnetic east or west, then nearly the maximum of deviation will be produced, for the iron of the ship will draw the needle towards the point p, and in a direction perpendicular to the line of the attraction of the earth.

When the ship is in harbour, place her head in the magnetic north, and take the bearing of some object; this bearing will be free from deviation, as the two attractions coincide: now warp the ship's head round till it be directed towards the magnetic east, and again take the bearing of the object; the difference between this bearing and the former one will be the deviation, when the ship's head is east: repeat this experiment in different positions of the ship's head with respect to the magnetic north, and note the deviations produced in each position; these are the deviations that will be produced at sea when the ship steers those particular courses: now take her compass on shore and place it on the top of a log of wood, in which a number of holes are bored at the distance of eight, nine, ten, &c. inches from the top; take also a circular plate of iron P, of about a foot in diameter, screwed on a brass leg, and fix it in one of these holes; turn the log round towards different points of the compass; and if the plate when fixed in that position with respect to the ship's compass, produces the same effect upon it at every point, that is produced on board the ship by her iron, the experiment is finished; if not, try different positions of the plate, until the precise situation is ascertained in which the same deviations are produced; then measure the distance of the centre of the plate below the card of the compass, and also its distance from the vertical line drawn through the centre of the card perpendicular to the earth, and take the compass on board again with the plate. It is found by experiment, that when the plate is fixed on board the ship in a position with respect to the compass, similar to that it occupied on shore, when it produced corresponding deviations, the deviation on shipboard, in the different positions of the ship's head, is very nearly doubled; for very nearly the same deviations are produced by the plate, that were produced by it on shore; and those deviations were equal to the deviations produced by the ship's iron,

the attraction of which' is now added to the attraction of the plate.

The proper position of the plate, therefore, having been ascertained as above mentioned, or by the tables accompanying it, take a bearing of any object, then fix the plate in its position, and observe its effect on the ship's compass; allow a deviation equal in amount to the effect produced, but so that the bearing without the plate may be intermediate between the bearing with the plate, and the bearing taken as the true one. For example, suppose a headland to bear due west, affix the plate to the compass; suppose that the same headland now bears Wb N, that is, the plate produces a deviation of a point westerlybut whatever deviation the plate produces, the iron of the ship must have produced nearly the same deviation before the plate was attached-therefore the iron of the ship must have drawn the compass one point to the west of its true position; so that the true bearing of the headland, independent of the effect of both iron and plate, must be W b S.

Another method of fixing the plate has been tried with success.-This method consists in fixing it behind, or abaft, the compass, and in a position in which its attraction will produce opposite effects, and neutralize that of the ship's iron: indeed some late investigations by the distinguished mathematician, M. Poisson, would seem to show, that this mode of correcting the deviation is to be preferred to the other plan of discovering its amount, and allowing for it ; especially in all cases where the deviation is likely to be considerable. In fact, this mode of fixing the plate was adopted in the trials made on board H. M. S. Griper, and in Captain Parry's two last expeditions, in high northern latitudes, where the deviation was very considerable, and the success of the experiments was notwithstanding very great.

We close our treatise with a description of this simple and beautiful contrivance; it is the last improvement which genius has furnished to the practical navigator, and an invention of our own times. With respect to the subject of this our third Part, we may observe that the navigator is indebted to the science of astronomy for the most important branch of his art; and that the

Plates are sold by W. and J. Gilbert, 143, Lead

enhall street, accompanied by tables, which show the attractions of the plates for several positions.

† See Quarterly Journal of Science, vol, xix.

p. 128.

TABLE I.

Logarithmic Sines, Tangents, and Secants, to every Point and Quarter Point of the Compass.

A

TABLE II.

A Table of Corrections to be added to the Mean Middle Latitude to find the True Middle Latitude.

8 620 030 040 060

O

8 12 8 15 8 198 230 27 8 31 8 35 8 40 8 45 8 518 58 1 66 I 14

0 020 030 040 060 09

0 020 03 0040 060 08

0 12 0 11

0 15
0 14

0 18 0 22 0 260 30 0 34 0 380 43 0 490 56 1 031 11 0 17 0 210 25 0 28 0 320 37 0 42 0 480 541 011 08

0 020 030 040 060 07 10 0 020 03 0040 060 070 09 0 620 030 04 006 007 0 09 230 020 03 0040 060 070 09 0 020 030 04 006 007 0 09

09

0 020 030 04 0 050 070 09 0 11
0 02 003 004 0 05 0 07 0 09 0 11

0 14

0 16

0 020 030 040 050 070 09 0 11
0 020 030 040 050 070 09 0 11

0 260 300 340 380 420 460 51 0 270 300 340 380 43 0 47 0 52 0 14 0 16 0 19 0 23 0 270 310 350 390 43 0 47 0 52 0 14 0 160 19 0 230 270 310 350 390 440 480 53 0 020 030 040 050 070 09 0 11 0 14 0 16 0 20 0 23 0 270 310 350 400 440 49 0 54 0 020 030 040 050 070 09 0 11 0 14 0 16 0 20 0 23 0 270 310 350 400 450 50 0 55 0 020 030 040 050 070 09 0 11 0 14 0 17 0 210 24 0 280 320 360 410 45 0 510 57

0 21 0 24 0 280 320 360 410 460 520 58
0 210 240 280 320 370 420 47 0 530 59
0 22 0 250 290 33 0 37 0 42 0 480 54 1 00
0 22 0 250 290 330 38 0 43 0 490 551 01
0 23
0 440 500 56 1 02

0 19

0 23
0 23

50

0 26 0 300 340 39

0 15

0 16

72

0 20 0 14 0 17 0 20 0 14 0 17 0 21 0 15 0 18 0 22 0 02 003 004 0060 09 0 12 0 19 0 23 0 27 0 310 35 0 400 46 0 020 030 050 070 09 0 12 0 15 0 19 0 23 0 27 0 31 0 36 0 41 0 47 0 541 011 081 15 0 020 030 05 0 07 0 09 0 12 0 20 0 240 28 0 32 0 370 42 0 49 0 561 03 1 10 1 18 0 020 040 050 070 09 0 13 0 16 0 20 0 24 0 29 0 33 0 390 44 0 510 581 051 121 21 64 0 020 04 0 06 0 0S 0 09 0 13 0 17 0 21 0 25 0 29 0 34 0 40 0 46 0 53 1 001 07 1 14 1 24 0 020 040 06 008 0 10 0 13 0 17 0 21 0 25 0 30 0 35 0 410 480 551 021 09 1 17 1 27 0 020 040 06 0 08 0 10 0 14 0 18 0 22 0 26 0 310 37 0 43 0 50 0 581 051 12 1 211 31 0 020 040 060 08 0 11 0 15 0 18 0 23 0 27 0 33 0 38 0 450 53 1 001 071 161 25 1 35 0 020 040 06 0 08 0 11 0 15 0 19 0 24 0 28 0 340 40 0 480 55 1 021 101 19 1 30 39 69 0 020 050 06 0 09 0 12 0 16 0 200 250 30 0 36 0 44 0 50 0 581 05 1 181 23 1 34 1 44 0 030 050 060 090 13 0 17 0 210 26 0 310 380 44 0 52 1 001 081 17 1 28 1 391 50 0 040 060 070 09 0 13 0 18 0 22 0 270 33 0 400 46 0 551 031 12 1 221 32 1 441 56 0 040 060 080 100 14 0 19 9 230 290 35 0 42 0 49 0 581 06 1 161 271 381 502 04

0 300 350 40 0 45 0 510 57 1 03 0 240 27 0 310 360 410 460 520 58 1 04 0 240 28 0 32 0 370 42 0 480 54 1 00 1 06 0 250 290 330 38 0 44 0 26 0 30 0 340 39 0 45

0 500 551 021 08

0 510 571 04 1 10

0 520 591 061 13

When the difference of latitude is under 2o, the correction may be neglected; when it is 2o and under 3o, ad!

correction of 0° 01′.

D

EXPLANATION OF SCIENTIFIC TERMS

MADE USE OF IN THIS VOLUME.

N.B. Many of the terms are common to this and the two preceding volumes; but those explanations that were given in the former Glossaries will be merely referred to in the present, except in a few cases, where the definitions were not supposed to be sufficiently explicit. Several omissions may be supplied by consulting the Indexes.

ABERRATION. The name given to an ap-
parent change of place in the fixed stars,
consequent upon the time taken up during
the passage of a ray of light to the eye
compared with that of the earth's annual
motion. This is familiarly explained in
the preceding glossaries, and more scien-
tifically in the present volume:—Astro-
nomy, pages 64 and 146, and History of
Astronomy, page 89.
ACCELERATION OF MOTION.-See
Glossary I. Acceleration may be either
regular or variable, that is, by equal or by
unequal accessions in equal times. The
motions of the planets are continually
accelerated, according to a known law,
from their aphelion to their perihelion;
while there is a continued retardation in a
like ratio in the opposite half of their
orbits the average of these motions
through the whole circuit (the space di-
vided by the time) is called the mean

motion.

ACOTYLEDONOUS.-See Cotyledon.
ACRONYCAL (a Greek compound denoting

the point of night) in its literal acceptation
denotes the moment of the sun's setting or
of his rising, the beginning or the end of
night. In modern astronomy it is confined
to the former; and a star is said to rise
or to set acronically when it rises or sets
at the instant of sunset. On the contrary,
when a star rises or sets in the morning
at the moment of sunrise, it is said to rise
or to set cosmically; but how this Greek
derivative has been so applied we have
not been able to determine.

In the case of its cosmical rising or setting, the star is never visible; but when it appears in the morning a little before the sun, or sets in the evening a little after him, it is said to rise or to set heliacally (Greek helios, the sun); and it is only in his heliacal rising or setting, that the planet Mercury is ever visible to the naked eye. ALMANAC.-See Calendar. The word is Arabic, signifying the reckoning. ALTITUDE of the sun, or of a star.-See

Glossary II., article Horizon; and Astronomy, page 16.

AMPLITUDE of the sun, or of a star.See Glossary II., article Horizon; Astronomy, page 256; and Navigation, page 30.

ANGLE.-See Glossary I.

-OF ELONGATION.-See Elon

gation.
ANNUAL PARALLAX-See Parallax.
ANNULAR, an adjective, from the Latin
annulus, a ring. Those eclipses of the
sun, in which a ring of light is visible
around the dark body of the moon, are
termed Annular Eclipses; they are also
Central Eclipses, because the centres of
the sun and of the moon appear to coin-
cide.

ANTARCTIC.-See Arctic.
ANTIPODES (Greek anti, against, and
podos, a foot). If a straight line be sup-
posed drawn from any point of the earth's
surface, through its centre, so as to ter-
minate in a point on the opposite surface,
two persons standing on those points
would be Antipodes to one another.
APHELION.-See Glossary II. and Astro-
nomy, page 127.
APOGEE.-See Glossary II. and Astro-
nomy, page 19.

APPARENT TIME.-See Time.
APSIDES.-See Orbit.

ARC OF A CIRCLE.-See Glossary I.
Angle.

ARCTIC. The Greek arctos, a bear, is the
origin of the two scientific adjectives
Arctic and Antarctic (anti-arctic) which
are equivalent to northern and southern.
An imaginary line passing through the
centre of the earth, and on which it turns
in its diurnal rotation, is called its axis;
and the two ends of that line, where they
are supposed to terminate at the surface,
are the Poles or pivots (Latin poli), and
are, respectively, the north and south (the
Arctic and Antarctic) poles of our globe.
If we suppose the line to be extended in
both directions, it will become an axis