When the given place is not a Standard Port for Reference, and both Constants with respect to such are not given. RULE LXXXIX. 1o. To find the Constant for Time.-Seek out that one of the Standard Ports for Reference which is nearest the given place. Next, take out the Time of High Water, Full and Change, for the Standard Port and for the given place (Admiralty Tide Tables, pages 131-234) and take their difference, which mark + (add) if the Full and Change at the given place is greater, but (subt.) if it is less than the Standard Port. 2o. To find the Constant for Height.-Proceed as directed in Rule LXXXVIII, page 310. 3°. Next, by the usual method find the time of high water nearest to the time of sounding, as given in the question, and proceed as in Case II, Rules LXXXVII and LXXXVIII. EXAMPLE. Ex. I. 1885, November 2nd, at 2h 15m A.M., mean time at ship, off Ile de Bas, took a cast of the lead: required the correction to be applied to the depth obtained by lead-line before comparing it with depth marked on the chart. Here the Standard Port for Reference is BREST, and both Constants for time and height must be found. At page 196, Admiralty Tide Tables, we find time of high water, full and change, at Brest to be 3h 47m, and spring range 19 ft. o in.; again, at page 194 of same tables, the time of high water, full and change, at Ile de Bas is found to be 4h 49TM, and spring range 23 ft. 0 in.; we next take difference of times of full and change, which is 1h 2m, which we mark +, because full and change, Ile de Bas, exceeds that at Brest. The difference of the spring ranges is 3 ft. 0 in., and is marked +, because the spring range at the given place is greater than that at Brest. The remaining part of the work as Case II. Brest, time H.W., full and change 3h 47m page 196. Constant 4 49 +1 2 194. Spring range 19 ft. 0 in. Constant Half mean spring range, Ile de Bas, = (23 ft. o in. 2) 11 ft. 6 in. 23 +4 2 15 A.M. I 45 3 12 91 inches. I 15 2)75 Course 37° and distance 91 gives departure 55'44 ft. 7 in. to add to half spring range, because time from high water is less than 3 hours. 1. 1885, June 16th, at 5h 22m A.M., mean time at ship, off Brest, took a cast of the lead. 2. 1885, June 9th, at 7h 18m P.M., mean time at ship, off Portsmouth, took a cast of the lead. 3. 1885, June 19th, at 6h 38m A.M., mean time at ship, off Harwich, took a cast of the lead. 4. 1885, Oct. 23rd, at 11h 18m P.M., mean time at ship, off Sunderland, took a cast of the lead. 1885, June 5th, at 5h 2m A.M., mean time at ship, off Beaumaris, took a cast of the lead. 6. 1885, June 1st, at 3h 32m P.M., mean time at ship, off Lamlash, took a cast of the lead. 7. 1885, June 5th, at 7b 27m P.M., mean time at ship, off Scarbro', took a cast of the lead. 8. 1885, August 11th, oh 50m P.M., mean time at ship, off Lerwick, took a cast of the lead. 9. 1885, June 20th, at 3h 33m P.M., mean time at ship, off Coleraine, took a cast of the lead. (NOTE.-Look in page 200 and 213 of Admiralty Tide Tables for the time of H.W, at Coleraine, full and change, and spring rise). MAGNETISM. MAGNETIC ATTRACTION AND REPULSION. 214. Magnetism.-Magnetism is that branch of physical sience which treats of the nature and properties of magnets and of their action on each other. The term magnetism is also used to designate the force which produces the effects called magnetic. 215. Natural Magnets,* or Lodestones.-Natural magnets, or lodestones, are exceedingly rare, although a closely allied ore of iron, capable of being strongly acted upon by magnetic forces, and hence called magnetic iron ore (known to mineralists as magnetite), is found in large quantities in Sweden, Norway, Spain, Arkansas, the Isle of Elba, and elsewhere. This mineral-the lodestone of the ancients-is composed of the peroxide and the protoxide of iron in combination with a small quantity of earthy substance having the chemical composition Fe, O. It is a dark gray colour, and the fresh fracture possesses a metallic lustre. curs in crystals, the usual form being the octrahedron. It frequently oc 216. Artificial Magnets.-When a piece of steel, which has been properly hardened and tempered, is rubbed with a piece of lodestone, or by any other magnet, or when a current of electricity is passed round it by means of a coil of insulated wire, it becomes a more or less powerful magnet: such a magnet is termed an artificial magnet. Artificial magnets are permanent or temporary as they retain their magnetism or not, after being removed from the source by which they acquired it. The most usual form of artificial magnets is that of a bar or horse-shoe. When single they are termed simple magnets, when consisting of several joined together, so that their similar poles are adjacent, they are termed compound magnets: the latter are sometimes designated magnetic batteries. A magnetic battery is, therefore, simply a bundle of magnets with their similar ends placed together. A Bar Magnet is simply a bar of cast iron or steel properly hardened, tempered, and magetised. The needle or bar of steel in the Mariner's Compass is an artificial magnet. The term Magnet (Magnis Lupis) was given by the ancients to certain black stones found in various parts of the world, notably at Magnesia in Asia Minor, which possessed the property of attracting small pieces of iron and steel. This magic property as they deemed it, made the magnet stone famous; but it was not until the tenth or twelfth century that such stones were discovered to have the still more remarkable property of pointing north and south when hung up by a thread. This property was turned to advantage in navigation. Its various names are interesting. In Pliny's time it was called ferrum vivum, quick iron; the Chinese know it as tchu-chy, the directing stone; and similarly in the Swedish it is segel-sten, the seeing stone; in Islandic, liederstein, the leading stone; and in English the lodestone (from Sax. to lead). ន ន 217. Poles and Neutral Line.-The property of attracting iron is very unequally manifested at different points of the surface of a magnet. If, for example, an ordinary bar-magnet be plunged in iron filings these become arranged round the ends of the bar in feathery tufts which decrease Fig. 63. towards the middle, which is bare, as in the lower diagram, Fig. 63. If the magnet is very thick in proportion to its length we may have filings adhereing to all parts of it, but the quantity diminishes rapidly towards the middle, as in the upper diagram, Fig. 63. The name poles is used in a somewhat loose sense to denote the two terminal portions of a magnet, or to denote two points, not very accurately defined, situated in these portions, and where the attraction is greatest. The middle portion, where there is no visible magnetic force, and to which the filings refuse to adhere, is called the neutral line. 218. Axis and Equator.-The shortest line joining the two poles is called the axis of the magnet: in a horse-shoe magnet the axis is in the direction of the keeper. The plane at right-angles to the axis and passing through the neutral line is called the equator of the magnet. 219. Consequent Poles.-Every magnet, whether natural or artificial, has two poles and a neutral line; sometimes, however, in magnetising bars and needles poles are produced lying between the extreme points. These intermediate points are called consequent poles. 220. Magnetic Needle.-Any magnet freely suspended near its centre of gravity is usually called a magnetic needle. One of its most usual Fig. 64. forms is that of a very elongated rhombus of thin steel, having very near its centre a concavity or cup, by means of which it can be balanced on a point. When it is thus balanced horizontally it does not, like an ordinary piece of matter, remain in equilibrium in all azimuths,* but assumes one particular direction, to which it always comes back after displacement. In this position of stable equilibrium one of its ends points to magnetic north and the other to magnetic south, which differ in general by several degrees from geographical (or true) north or south. This is the principle on which compasses are constructed. All lines in the same vertical plane are said to have the same azimuth. Azimuthal angles are angles between vertical planes, or between horizontal lines. The azimuth of a line, when stated numerically, is the angle which the vertical plane containing it makes with a vertical plane of reference, and this latter is usually the plane of the meridian. Some readers may be glad to be reminded that by the plane of the meridian is meant a vertical plane passing through the place of observation, and through or parallel to the earth's axis. A horizontal line in this plane is a meridian line. 221. Mutual Action of Poles.-The two poles of a magnet appear identical when they are brought in contact with iron filings, but this indentity is only apparent. On presenting one end of a magnet to one end of a needle thus balanced we obtain either repulsion or attraction, according as the pole which is Fig. 65. presented is similar or dissimilar to that to which it is presented. For when a small magnetic needle ab (Fig. 65) is suspended by a fine thread and the north pole, A, of another needle is brought near its north pole, a, a repulsion takes place. If, on the contrary, A is brought near the south pole, b, of the movable needle the latter is strongly attracted. Hence, these two poles, a and b, are not identical, for one is repelled and the other attracted by the same pole of the magnet A. It may be shown in the same manner that the two poles of the latter are also different by successively presenting them to the pole, a, of the movable needle. In one case there is repulsion, in the other attraction. Hence the following law may be enunciated : Poles of contrary names attract one another, and poles of the same name repel one another. But further, the forces exerted by the two poles of the same magnet are equal as well as opposite. This may be shown by bending a magnet so as to bring the opposite poles nearly into contact. The action of the two poles. upon a distant point will be found to neutralise one another. The opposite actions of the north and south poles may be shown by the following experiment:-A piece of iron, a key for example, is supported by a magnetised bar. A second magnetised bar of the same dimensions is then Fig. 66.-Showing the mutual action of two dis-similar poles in lessening the PORTATIVE and moved along the first so that their poles are contrary. The key remains suspended so long as the two poles are at some distance, but when they are sufficiently near the key drops, just as if the bar which supported it had lost its magnetism. This, however, is not the case, for the key would be again supported if the first magnet were presented to it after the removal of the second bar. The attraction which a magnet exerts upon iron is reciprocal, |