Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/288

276 NAVIGATION there be sea room, across the direction of the wind, from the centre of the storm, that is, with the wind on the starboard side in north latitude, and on the port side in the southern hemisphere. If it be necessary to scud, it should be done with the wind as much on the starboard or port quarter as the ship will bear, in order to avoid running round or into the centre. The wind does not run in parallel circles round the centre, but rushes spirally towards it, especially when close (see fig. 22). There is one dangerous position in which a ship may be placed with respect to an advancing cyclone for which no certain rules have been given, such as at A or D, or between those two posi tions, near the track upon which it is advancing. It is evident from the diagram that to run from A either before the wind, or with it on the starboard quarter, in the direction of the small arrow, would cause the ship to inter cept the storm s centre. It would be advisable to carry sail with the wind abeam as long as pos sible, and nlti- mately to heave FlG " 22.-Cyclone in 30 N. lat. to on the starboard tack (in north latitude) till the barometer begins to rise, and the wind turns more to the south and west. As the centre of the storm is supposed to move at a rate varying from 12 to 30 miles an hour, a ship sailing 10 or 12 miles an hour in a direct line from the centre, but on the track, would be overtaken by it. The great hurricane of 1830 extended from the West India Islands to Newfoundland at a mean rate of 18 knots an hour, and by the average time occupied in passing in termediate places (eighteen hours) it must have been about 330 nautical miles in diameter. The centre of the hurricane which devastated Barbados August 10, 1831, moved at a mean rate of about 14 knots an hour. It is evident from those quotations that a ship cannot depend upon running faster than the storm. The navigator should be ready provided with a representation on tracing paper of a cyclone similar to fig. 22, of a radius equal to 150 miles on the scale of the chart upon which it is to be used. If made for one hemisphere it can be turned over to suit the other. By marking the ship s position on the chart, and placing the tracing over it with one of the arrows correctly corresponding with the direction of the wind at the ship, the direction of the storm s centre will be indicated with certainty, but the distance and track can only be conjectured, the latter by assuming that it will be similar to those previously ascertained, which may be represented by a pencil line on the chart. If to the south of the 20 N. lat. it will be moving in a westerly direction, if about 30 to the north west or north, and when about 35 and 40 to the north-east. It is safer to assume the centre to be nearer than it really is than vice versa ; the pencil line will run parallel to the storm s course unless the ship be on the track, as at D. It is evidently her duty to run to the west if there be room till the wind veers to the north east against the sun, when it will be certain that she is to the west ward of the storm s track. This is not likely to happen in the North Atlantic when as far north as 30, as the centre usually passes along the coast or over the continent of America ; but when farther south, and when the storm s progress is to the westward, a ship may escape by running to the southward with the wind at north. If the position with regard to the storm is supposed to be at B, she should run with the wind on the starboard quarter. If at C it will be optional either to run with the wind on starboard quarter or to heave to ; for anything in that direction relative to the assumed track will have attained its nearest approach to the centre. When the direction of the wind remains unchanged, the force increasing and barometer falling, the centre must be advancing directly on the ship. AVhen the centre begins to recede the wind will decrease in violence and change its direction, with the sun if the ship is east of the track and against the sun if the centre passed to the east ward of the ship ; the barometer will also rise rapidly. Fig. 22, and the argument upon it, are for the northern hemi sphere ; the typhoons and all circular storms south of the equator are similar, but the revolutions are reversed, and the track of centre curves off from west to south-west, south, and south east. 1 In ordinary heavy gales in the North Atlantic a ship should heave to on the starboard tack, as the wind generally changes from south to west and north-west. On that tack she will be constantly " coming up" and bowing the sea as the wind veers, and will be less liable to be taken aback; and for similar reasons the third anchor for use should be carried on the starboard side. The reverse will hold good south of the equator. The storm peculiar to the west coast of Africa is called a tornado, It is entirely different from a hurricane ; it is not circular; it is very local and of short duration, seldom lasting more than ten or twenty minutes. These storms give good warning by a black cloud first appearing on the horizon, rising quickly. When the cloud is about twenty degrees high a bright arch will be seen under it. The faster it rises and the more distinct the arch the more severe will be the tornado, which bursts suddenly when at an angle of forty-Jive degrees. During its rise there will be ample time to shorten all sail and turn the ship s stern towards it ; she will be driven violently for a short time, when it will fall calm as suddenly as the wind came, and a deluge of rain will wash the decks. Along the coast of California and the west coast of Mexico and Central America voyages can be made to the south-east with westerly and south-westerly winds and favourable current, except during the latter part of May, June, and July, when the trade winds are strongest through the West India Islands ; the wind is then off shore at the southern part of the track mentioned, and easterly, and passages along shore to the south-east are not easily accomplished. With the exception of the time last named, when ships are bound from Central America to California they must stand out into the trade wind, run down the meridian, and then make northing. Along the west coast of South America light southerly winds and northerly current prevail, therefore ships bound south from Central America should stand well out from the land. On the southern part of Chili south-west winds prevail. On the west side of North America, beyond the tropic, westerly winds are most frequent, as in a similar belt across the Atlantic. Ships bound to the East Indies will find, after rounding the Cape of Good Hope, the south-west monsoon from April till October giving them a fair wind and fine weather through the Mozambique to Bombay or Calcutta. The reverse is the case during the English winter months, that is, from October till April. In the Indian Ocean, as in the northern hemisphere, the most unsettled and stormy weather is about the time of each equinox, but more especially about the end of September, when circular storms of incredible violence called typhoons are sometimes experienced. When south of the equator, ships should always receive them on the port side, or run with the wind on the port quarter, if there be sea room; if north of the equator, keep the wind on the starboard side, as described above. Bad weather is frequent on the coast of Malabar during May, and in the Bay of Bengal during October. When running up the Bay of Bengal in May or June before a south-west gale, with heavy rain and falling barometer, there will be reason to fear being overtaken by the centre of a typhoon ; it will be advisable to heave to, or run to the eastward, till the barometer rises and weather improves. In the Red Sea the wind is generally from the south, except dur ing June, July, and August, when it is north, calm, or on shore by day, and off shore at night. From the Cape of Good Hope to Australia and New Zealand ships have usually westerly winds and a favourable current, which is a good reason for returning to Europe by way of Cape Horn. About 60 to 100 miles off the south-east extremity of Africa the Agulhas current sets to the westward, sometimes as much as 60 miles a day; its progress is not checked by a westerly gale, and con sequently one of the most dangerous seas ever experienced is to be found there. About the year 1850, ships making long voyages were frequently navigated upon or approaching a great circle, which led the Australian ships into danger on account of the icebergs which were met in great numbers. In 1866 the Admiralty published ice-charts of the southern hemisphere for the guidance and warning of seamen. Every ship should be provided with wind and current charts for the sea to which she is going, and every officer should help to perfect these by means of careful observations. When a ship is to leeward of her port, or a chase, it is advisable to work up on a direct line towards it, tacking as frequently as con venient, as then all flaws and changes in the wind will be in her favour. By the contrary mode of action ships have been known to sail round an island, e.g., the Bermudas. A useful graphic method has been devised for shaping a course which will intercept another ship with the least loss of time, or for shaping a course across a current the direction and velocity of which can be estimated (see fig. 23, which for use should be more than twice as large, with a circle for every mile). 1 For full description of circular storms, see Lieut.-Col. V. Reid, Law of Storms, 1S50 ; Henry Piddington on Typhoons, Calcutta, 1840 ; and W. H. Kosser, The Law of Storms, &c., 1870