34. Observed Altitude is the observed angular height of an object above the visible horizon.

35. Apparent Altitude is the apparent angular height of an object's centre above the sensible horizon.

36. True Altitude is the true angular height of an object's centre above the rational horizon.

37. Civil Time is used to record events in ordinary life. The day commences at midnight and terminates at the following midnight. It is divided into two divisions of 12 hours each, the first being termed A.M. (ante meridiem) or before noon, the latter P.M. (post meridiem) or after

noon.

38. Astronomical Time is used in all astronomical calculations. The day commences at noon and terminates at the following noon, without the distinction of A.M. and P.M.

39. Sidereal Time is reckoned from the passing of the first point of Aries across the meridian. The day is consequently the time which the earth takes to make one revolution on its axis, viz., 23 н. 56 м. 4·09 s.

40. Apparent Time is the angle at the pole, between the meridian which passes through the sun and the meridian of the observer, reckoned westerly.

The Apparent Solar Day is consequently the time which elapses, between the departure of the sun from the meridian of a place (or apparent noon) and his return to the same meridian; it is a period of unequal duration, because the sun moves in the ecliptic and not in the plane of the equator, and thus causes a daily change in his position relative to the earth which sensibly affects his apparent velocity.

41. Mean Time is a uniform period used for convenience, on the supposition that the sun moves in the plane of the equator instead of in the ecliptic.

The Mean Solar Day is consequently the average or mean of all the apparent solar days in the year.

42. Equation of Time is the difference between apparent and mean time.

43. Variation of the Compass is the angle the magnetic meridian makes with the true meridian.

44. Deviation of the Compass is the angle the compass needle makes with the magnetic meridian.

45. The Error of the Compass is the angle the compass needle makes with the true meridian.

46. True Course is the angle a ship's track makes with the true meridian.

47. Magnetic Course is the angle a ship's track makes with the magnetic meridian.

48. Compass Course is the angle a ship's track makes with the compass needle.

49. Leeway is the angle a ship's track makes with the fore and aft line of the vessel.

50. The Complement of an arc or angle is the quantity by which the angle differs from a quadrant or 90 degrees.

51. The Supplement of an arc or angle is the quantity by which the angle differs from a semicircle or 180 degrees.

TERRESTRIAL MAGNETISM.

The Earth represents an enormous magnet whose two poles were in 1835 respectively situated in latitude 70° 5' N., longitude 96° 45′ W., and latitude 73° 30′ S., longitude 147° 30′ E. These magnetic poles travel round the true poles of the earth at a radial distance of nearly 1,200 miles and carry along with them the entire system of terrestrial magnetism in a direction contrary to that in which the earth revolves, the system performing one entire

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revolution in every 960 years; thus, as the magnetic needle on the meridian of Greenwich pointed to the true north pole of the earth in the year 1659, it follows that it will be on the intermediate 180th meridian in the year 2,139, and again on that of Greenwich in the year 2,619; hence the continual change in the variation of the needle as shown on the Chart of the Curves of Equal Magnetic Variation, published by order of the Admiralty. Now, as poles of contrary names attract, and of similar names. repel one another, it follows that the north magnetic pole of the earth and the north pole of a magnetic needle cannot both contain north magnetism, because they are attracted towards one another; therefore, to avoid confusion, that pole of an artificial magnet which is attracted towards the astronomical north is said to contain south or red magnetism, and that pole which is attracted towards the astronomical south is said to contain north or blue magnetism.

Thus, the North magnetic pole of the earth contains north or blue magnetism, and the South magnetic pole contains south or red magnetism.

The Magnetic Meridian of a place is a vertical plane which passes through the two poles of a magnetic needle at the place.

The Variation or declination of the magnetic needle is the angle which the magnetic meridian makes with the true meridian of a place.

The Dip or inclination of the needle is the vertical angle which the magnetic needle makes with the plane of the horizon.

The Magnetic Equator, or line of no dip, is a somewhat sinuous line near the equator and crossing it in two places where the needle is horizontal: this line constitutes the division between the Northern and Southern magnetic hemispheres.

North of the magnetic equator the red end of the

dipping needle is pulled downwards, and the blue end repelled upwards, gradually increasing the inclination with the increase of the latitude, till at the magnetic pole the dip is 90 degrees.

South of the magnetic equator the blue end of the needle is depressed, and the red end elevated, till at the south magnetic pole the needle is again vertical.

The sun has considerable influence on the magnetism of the earth, inasmuch as the magnetic force is greater in both hemispheres in the months of December, January, and February, when the sun is nearest to the earth, than in the months of May, June, and July, when more distant. The mean solar parallax is 8.760 seconds, which corresponds to a mean distance of 93,300,000 miles.

The daily passage of the sun across the meridian has a perceptible influence on the magnetic needle; thus, the red end of the needle travels from east to west with the sun, from sunrise till nearly 2 P.M., from which time till about 10 P.M. it gradually recovers itself. The mean of these diurnal variations throughout the year in the latitude of the English Channel is about 12 minutes, though they have been known at Paris to reach 25 minutes, and sometimes in the Arctic regions even 2 degrees; near the equator the oscillations entirely cease.

The Aurora Borealis and the Aurora Australis both cause variations of the needle, which sometimes in our latitudes amount to 20 minutes, while in polar regions the needle frequently oscillates several degrees; its irregularity is considerable on the day before the occurrence of the phenomenon.

Magnetic storms also exercise considerable influence on the needle: it has been established by Sabine that these disturbances attain their greatest frequency about every ten years, and that those periods of maximum magnetic disturbance coincide with the periods when sun spots attain their greatest number and size.

DESCRIPTION OF THE STANDARD COMPASS.

The Standard Compass is the only compass by which a ship should be navigated. It should be used for azimuths, amplitudes, stellar observations, and in taking bearings of the land. All other compasses, such as the steering compass, bridge compass, and mast-head or tripod compass, are to be considered only as supplementary to the standard compass.

In selecting a position for this compass, the vicinity of vertical iron, or of any considerable mass of iron, must be guarded against; and no portable pieces of iron subject to occasional removal permitted to be placed within a radius of say 15 feet. The standard or column must be permanently fixed in the middle line of the vessel as near to the neutral point of local magnetic attraction as can conveniently be arranged. The advantage of ascertaining a neutral point, where the local attraction is scarcely felt in any direction of the ship's head, is well worth the trouble of experiment.

In an iron ship, built head north, the standard compass ought to be placed as far forward as convenient: if built head south it should be placed as far aft as convenient, but not too near the rudder-post or steering-gear; if built head east or west it should be placed nearly amidships.

The compass-bowl should be made of pure copper of considerable thickness, and placed at such a height above the deck that bearings of distant objects can be observed over the bulwarks or deck-houses with perfect ease.

The two needles must be fixed parallel to one another under the card at a distance of from 50 to 60 degrees apart, that is, the extremities of each needle should be from 25 to 30 degrees from the north and south points. of the card respectively. They should be deep vertically, but narrow, in order that the direction of magnetic force coincide with their middle lines.

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