網頁圖片
PDF
ePub 版

Compass.

drawn on the importance of the functions of the organ in the economy. Moreover, with a knowledge of comparative anatomy, the physiologist can conduct experiments on animals which have organs similar in structure to those of man, and determine their functions more precisely than would be possible in the human body. To the veterinarian a knowledge of the comparative anatomy of the domestic animals is essential to the study of their diseases. To the embryologist, a knowledge of the anatomy of different animals throws light on the signification of the structural changes which the body of any particular animal passes through in the course of its development. To the zoologist, a knowledge not only of the external form but of the internal structure of animals is essential in order that he may frame a precise system of classification. In the present work the anatomy of the different classes and of some of the more important orders of the animal kingdom is arranged under special heads.

COMPARISON, in grammar, and as applied to adjectives (q.v.), is that which marks the degree in which the quality is attributed to the object, as compared with other objects. There are three degrees of comparison. The positive indicates the quality generally, without comparison; the comparative, a higher degree of the quality than is attributed to other things; and the superlative, the highest degree that is attributed to any of the things under consideration. There are two ways of expressing these degrees. 1. By an inflection or change on the word; as, hard, harder, hardest; happy, happier, happiest. This mode prevails almost exclusively in Greek and Latin. 2. By an additional word, as more happy, most happy. This may be called logical C., the other, grammatical. In French, with the exception of a few irregular adjectives, all adjectives follow the logical method. In English, the logical method is preferred in every case where the grammatical would produce a word difficult or harsh in the pronunciation. This is generally the case in English when the simple adjective is of more than one syllable; but it is not always so. Thus, earnester, prudenter, would make harsh combinations; not so politer, discréeter, happier. The difference is, that in earnester, prúdenter, the accent being on the first syllable, two unaccented syllables of discordant character are thrown together; in politer, discréeter, the unaccented syllables are separated; and in happier, though they come together, they are of a kind readily to coalesce. Thus, the laws of euphony-which mean, the ear and organs of speech consulting their own convenience-determine this point, as they do much else in language. In general. it is only adjectives of quality that admit of C.; and even adjectives of quality cannot be compared when the quality does not admit of degrees; as, a circular spacea, a gold ring,

a universal wish.

Adverbs (q.v.) are compared exactly like adjectives.

COMPASS, MARINER's, is the name given to the instrument by which sailors are enabled to steer their course on the ocean out of sight of land, and when neither sun nor stars are visible. The ancients, to whom it was unknown, could seldom venture to lose sight of the coast. The directive power of the magnet seems to have been unknown in Europe till late in the 12th century. It appears, however, on very good authority, that it was known in China, and throughout the east generally, at a very remote period. The Chinese annals indeed assign its discovery to the year 2634 B. C., when, they say, an instrument for indicating the s. was constructed by the emperor Hou-ang-ti. At first, they would appear to have used it exclusively for guidance in traveling by land. The earliest date at which we hear of their using it at sea is somewhere about 300 A.D. According to one account, a knowledge of the C. was brought to Europe by Marco Polo, on his return from his travels in Cathay. It was long contended that the C., as a nautical instrument, was first invented by Flavio Gioja, a native of Amalfi, about the year 1362; and that that part of the kingdom of Naples where he was born has a compass for its arms. For this there is no authority whatever, as the C. was well known as a nautical instrument before his time. It is probable, however, that Gioja may have made some improvement in the instrument or in the mode of suspending it. See British Annual for 1837.

The essential part of the C. is the magnetized needle, balanced freely upon a fine point; such a needle has the property of arranging itself in the meridian, one end always pointing to the n., and the other to the s. (see MAGNETISM); yet not exactly, but with a deflection or declination (q.v.), which varies from time to time in magnitude, and may be towards the w. or the east. The form of the needle is various, some being long flat prisms, others lozenge-shaped. The center of the needle is pierced, and a piece of agate generally inserted, with a conical hole sunk in it, to receive the fine point of the steel pin, so that the free motion may not be hindered by friction. The construction of the rest of the instrument depends upon the purpose it is to serve. For a mariner's C., the needle is fixed to a circular card, on which there is a star of 32 rays marking the 32 points of the heavens. The n. point of the card is immediately over the n. end of the needle, and the card moves with the needle. The cardinal points are marked with the letters n., s., e., w.; and the intermediate points other letters. The points are often subdivided into quarters, which are thus marked: n.te. (read "n. one fourth e."); n.je.; n. by e.fe., etc. A point of the C. being of the circle, is equal to 11° 15'. The C. thus formed is inclosed in a cylindrical brass box; and in order that the C. may remain horizontal in all positions of the ship, the box is suspended

by gimbals (q.v.). The whole is then placed in the binnacle (q.v.), in sight of the helmsman. Inside the box, in the direction of the ship's bow, is a vertical black line called the lubber-line, and the steersman must keep the point of the card which marks the prescribed course always in contact with the black line. Compasses differing in some respects from the mariner's C. are also used in surveying and land-measuring; and for the miner they are perhaps as necessary as for the sailor.

The great difficulty connected with the use of the mariner's C. arises from the disturbing influence of the magnetism of the ship. The difficulty is particularly felt in iron vessels, where the deviation of the needle is frequently so considerable as to render the compass almost useless. Various means of obviating this have been suggested; one of these is to place bars of soft iron or magnets in the immediate neighborhood of the binnacle, which being so placed as to cause a contrary disturbance to that of the iron of the ship, leave the needle comparatively free. This is found to answer well in iron ships plying between British and continental or North American ports; but where, as in the Australian passage, they change considerably their latitude, such an arrangement is found to be worse than useless, as the magnetism of the vessel changing with the magnetic latitude, causes an ever-varying deviation of the needle. It has likewise been suggested to place a compass as a standard at the mast-head, where it would be comparatively free from the attraction of the vessel, by which the ship's course might be shaped, the ordinary C. being used merely to give immediate direction to the steersman. In the royal navy, this error is to a large extent obviated in the following way. A C. is placed so high above the deck as to clear the bulwarks, and allow the bearings of a distant object on shore or a heavenly body to be taken while the ship's head makes a complete circuit. In this way, the deviation caused by the iron of the ship in all dif ferent positions may be ascertained. Thomson's C., patented 1876, is the best.

[ocr errors]

COMPASSES, instruments for transferring and marking off distances, or for drawing circles, ellipses, etc. The common C. or dividers are simply two rods or legs" joined together at one end by a pivot-joint, and pointed at the other; when used for drawing circles, the lower part of one of the legs is replaced by a pen or pencil. Spring dividers are much in use by workmen; in these, the legs are united by a strong steel spring, the action of which is to stretch them open; but half-way down, a screw passing between the legs, regulates the degree of opening. The value of these depends upon the per manency with which they retain any degree of opening given to them, pivot C. being liable to slip.

Beam C. consist of points sliding on a long bar, to which they may be clamped at any distance from each other. They are used for greater openings than pivot C. can safely span, and, when delicately made, for more accurate dividing. See GRADUATION.

Proportional C. have a point at each end of each leg, and the pivot between, thus forming a double pair of C. opposite to each other, end to end. If the pivot is midway between the points, the opening of each pair of points will be equal; if the distance from one pair of points be double that from the other, the openings will be as two to one; and so on for any ratio. When a single fixed proportion only is required, the pivot is fixed accordingly; but to adapt them for variable proportions, the pivot is a clamping screw, which moves in an elongated interval in the legs, and may be fixed at any point.

Triangular C. have three legs, so that the points of a triangle may be all transferred at once.

Calliper Compasses. See CALLIPERS.

COMPASSIONATE ALLOWANCE is an allowance made to the children of deceased British military and naval officers left in necessitous circumstances. The annuities vary from £8 to £20 a year, according to the rank of the deceased officer; and are tenable up to the age of 18 in the case of boys, and to that of 21 as regards females, or until marriage if that happen earlier. This measure is more general than a C. A. which arose out of the sympathy of the British nation with the army during the Crimean war. Royal warrants were issued in 1855 and 1856, defining and explaining allowances made to the widows and children, or other specified relations, of military officers who, after the date of the declaration of war with Russia, had been killed in action, or had died of wounds received in that war. The claims on this fund die out as years advance, of

course.

COMPASS PLANT, called also "resin weed" because it abounds in resinous matter; a large plant growing on the prairies, whose leaves are said always to point directly or nearly n. and s. When cultivated in gardens this property does not always

appear.

COMPASS, SOLAR, an instrument for determining at any place an accurate n. and s. line. It has a range of about 35°, and may be adjusted to the latitude of any place in the United States. It has a latitude arc, a declination arc, and an hour arc, each to be duly adjusted for an observation; and has been found of much service in running important boundary lines, and other government surveys. One of its recommendations is its avoidance of the perplexities caused by local attraction. It is the invention of William A. Burt, of Michigan.

[ocr errors][merged small][merged small][graphic][subsumed][merged small][ocr errors][merged small][merged small][ocr errors][subsumed][subsumed][ocr errors][subsumed][subsumed][merged small][merged small][merged small][subsumed][merged small][merged small][merged small][merged small][graphic][graphic][subsumed][subsumed]

THE COMPASS, MAGNETISM, POLARITY, ETC.-1. Maguetic poles. 2. Magnetic rod (suspended). points. 6. Illustration of magnetic polarity. 7. Magnetizing iron. 8, 9. Natural magi distance. 13. Magnetization by friction. 14. Declination. 15. Apparatus for determi 18. Magnetic inclination. 19. Inclinatorium. 20. Map of inclination. 21. Diurnal variat compass. 27-30. Magneto-electric telegraph instruments of Siemens and Halske.

[graphic][graphic][ocr errors][subsumed][subsumed][graphic][graphic][subsumed][graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][merged small][subsumed][merged small][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][graphic][graphic][subsumed][ocr errors][subsumed][merged small]

3. Magnetic needle. 4. Magnetic needles with marked poles. 5. Magnetic staff, broken at various et, with armature. 10. Horseshoe magnet. 11. Magnet armature. 12. Action of a magnet at a ning the degree of declination. 16. Magnetism of the terrestrial sphere. 17. "Jacob's staff." on of inclination. 22-24. Magnetometer. 25. Magnetic variations or disturbances. 26. Surveyor's

« 上一頁繼續 »