Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/287

Rh NAVIGATION 275 cross in the centre of the circle upon which the ship was situated. Repeat the operation from the centre to the third object, and where the two circles cross will be the ship s position. The distance off shore can be ascertained by one such angle subtended by two known objects nearly abeam, which might be useful in keeping a ship outside or within a shoal. About 60 is a desirable spread. The distance from an island, the extremities of which do not spread sufficiently to give cross-bearings by the compass alone, or the height of a lighthouse, tower, ^or cliff, may be made useful in like manner. Tides. The ability to find the time of high water at any place during the interval between &quot;full and change &quot;was once highly estimated, but in the present day, when almanacs are so plentiful, the information can be had without trouble. As a fact a modern navigator never does calculate the time of high water, because the rule only gives a uniform rate of change for all places on the globe, which is more or less incorrect ; and, knowing the change which has taken place at London Bridge since the new moon by the A Imanac, he has but to apply it to the standard of any other place. The Nautical Almanac gives the mean time of two tides daily at London Bridge throughout the year, and the apparent time of high water on full and change days at about two hundred places in the United Kingdom and on neighbouring coasts. A more full account of the tides is published annually by the Admiralty. The times predicted in the Almanac are but approximations, and are fre quently half an hour too soon or too late. A ship under sail only, working up or down the British Channel, will derive great assistance from the tides by making short tacks close to the head lands and a line joining them in the greatest strength of the tide, during the time it is favourable, and standing out towards the centre of the channel during the adverse tide. Winds and Currents. To make good passages a navigator should possess a general knowledge of the prevailing winds and principal currents which he is likely to encounter whether he be under sail or steam. When in the vicinity of land he should always be on his guard against unexpected effects from currents, for none are accurately known or free from change at certain or rather uncertain periods. The strength and direction of ocean currents can only be ascertained by comparing carefully kept dead-reckoning with the result of frequent observations, both of which are apt to be neglected when the navigator is free from land. A great impedi ment to a better knowledge of the currents is the assumption that they are well known and that the same water circulates across and round the seas, as it would in an enamelled basin. There is decidedly an indraught on the north-west coast of Africa, in the Bight of Biafra, and in many other places. A navigator should therefore be on the watch when steering parallel to a coast at night. The advice here offered with regard to the choice of a track is intended for sailing ships only, steamships are more independent. On leaving England for Gibraltar or Madeira the prevailing winds and currents will be found to set along the coast of Spain and Portugal ; generally a passage to the south is easily effected. Westerly gales are frequently very strong ; therefore ships should, while able, shape a course well outside of Cape Finisterre, towards which there is frequently an indraught. Should bad weather threaten from the south the ship (unless convenient to a port) should stand to the westward to gain an offing, before the force of the wind is felt or it turns to the westward. She should then wear and heave to on the starboard tack, which will enable her to bow the sea as the wind changes to west and north-west. This is an invariable rule when north of the equator ; the reverse is the rule in the southern hemi sphere. Ordinary heavy gales frequently give warning of their approach by causing a set of the sea in the direction of their course, accom panied by a long swell, the day previous to the force of the wind being actually felt at the ship. The height of the waves from hollow to crest is sometimes over 40 feet. Ships bound to the West Indies usually call at or pass near Madeira, thence continue south past the Canary Islands till they fairly catch the north-east trade wind, which may be expected in 30 N. lat., or sooner about the month of July. A fair wind and current will then be secured to the vicinity of the West India Islands, where it either falls calm, or a light wind from the shore is experienced every night. When bound to ports in the United States it is necessary to keep to the southward in a similar manner, skirting the northern edge of the trade wind. From either of those places or the Gulf of Mexico the homeward track is the same, that is, close to the banks of Newfoundland, in order to get the prevail ing westerly winds. Ships bound up the English Channel should enter near the centre, avoiding Ushant and Scilly, especially in thick weather. Steam ships are more under command, and should be more accurately navigated, but the advantage thus gained in point of safety is more than counterbalanced by the unreasonable things required of them, such as communicating with the lighthouse on Bishop Rock, which caused the loss of the &quot; Schiller &quot; and many lives a few years back. The prevailing wind in the English Channel is south-west, and it will be found in the western half of the circle two days for everj one in which it is in the eastern half. Ships bound to Sierra Leone and the coast of Guinea should pass Madeira on either side, then outside or through the Canary Islands, and if it be about the month of March inside the Cape Verd Islands, where a strong north-east trade will be blowing. At other seasons pass outside these islands to avoid calms, thence close along shore with westerly and south-westerly winds. The current in this track will be found favourable from the Bay of Biscay to the Bight of Biafra, but not farther south. From Sierra Leone to Loando steer along the coast to Cape Palmas, then stretch across the south-east trade till able to return towards Africa on the opposite tack, which may be at 200 or 300 miles south of Ascension. It is especially necessary to keep well to the southward during July and August, when calms prevail near the shore. From Biafra to near the Cape of Good Hope the usual wind near the coast is from the south-west during the day, and either very light off shore or calm at night. From England to South America the track will be similar to that to Sierra Leone as far as the Cape Verd Islands, when ships for the north-east coast may bear away to the westward, but always allowing for the strong current which may set them to leeward of their port during the calms to which they will bo liable. Those bound to the southward of Cape St Roque must be careful not to make much westing till they have crossed the equator ; the exact place most desirable for crossing has long been disputed, though the positions generally advocated lie between the meridians 18 and 26. The argument in favour of the former is the certainty of weathering Cape St Roque, and for the latter less delay by calms between the two trade winds, i.e., in the space from about 10 north to the equator, or from 10 to 4 north while the sun is north of the equator; in this half of the year the S.E. trade comes farther north, and on the coast of Brazil it then blows from south-east and south, so that it is advisable to cross the equator at about 20 W. During December and January the wind frequently draws along the south-east coast of Brazil, past Bahia, from the E. N.E. At that season the line maybe crossed in 25 or 26. All these rules have their exceptions : when the wind deviates from the usual direction, and the deviation is unfavourable, it is unquestionably better to put the ship on the opposite tack till it returns to the usual direction. From the United States to the South Atlantic the course should be east, keeping to the north of Bermuda (32^) till past 40 W., then south-easterly across the north-east trade, and joining the track of ships from Europe. After passing the latitude of Cape St Roque, ships for Rio, Monte Video, or Cape Horn may steer direct courses, but for St Helena or Benguela they should keep to the port tack, standing to the south ward till past the twentieth parallel, or a day longer if the wind should then be to the eastward of south-east. If bound to the Cape of Good Hope we must be content with making southing only till past the latitude of Rio; westerly winds may then be expected. The track indicated by an arc of a great circle from Rio to the Cape is not only the shortest, but about the best with regard to the wind. When homeward bound from the Cape, ships should pass close to St Helena with a fair wind and current, and steer for about the western limit for crossing the line on the outward voyage, where they would meet the ships from the south-east coast of America, and running together across the centre of the Atlantic join the homeward bound West Indiamen between the Azores and New foundland, according to the winds experienced. From the Bight of Benin or any part of the Guinea coast it is necessary to stand south on the starboard tack to within a degree of the equator, then due west till past the meridian of Cape Palrnas, and north-west into the middle of the Atlantic. In the Mediterranean the prevailing wind is west, therefore there is no difficulty in running up that sea. A slight current sets from all directions towards Cophalonia. Off the coast of Algeria, during westerly gales, heavy hollow waves are experienced, running at the rate of 10 or 12 miles an hour. When making a passage out of the Mediterranean it is better to keep near the African coast while the wind is to the southward of west, to which it is sure to turn, and then stand over to the Spanish coast, where shelter can be found if required. The destructive circular storms or cyclones in the West Indies called hurricanes are generally preceded by calm gloomy weather and a high barometer. It is very desirable that the electric state of the atmosphere and the earth should be known, which is not the case at present. Their immediate approach is indicated by the barometer falling rapidly ; which is the more remarkable, because the changes in the barometric readings are usually very slight within the tropics. If the ship be in a safe anchorage or harbour, the lower yards and topmasts should be struck and anchors laid out in the direction of the most probable danger. If the anchorage be not satisfactorily protected the sooner she is out of it the better. At sea it is necessary to get down the upper spars, and to run, if