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Appendix A.

THE LAW OF STORMS IN THE EASTERN SEAS.

INTRODUCTION.

The first chapter of the following paper treats of the law of storms and was first published in Hongkong in September, 1886. It was read before the British Mercantile Marine Officers' Associa- tion on the 17th May, 1893. It is now reprinted with later additions. The second chapter treats of the management of ships in typhoons, and was originally included in the pamphlet on the law of storms. It was read as a separate lecture before the Shipmasters' Society, London, on the 16th January, 1896, and is now reprinted with a few additions. The third chapter was printed in the Government Gazette. The fourth and fifth chapters, on different classes of typhoons and on winter- typhoons in the southern part of the China Sea, appeared in "Zeitschrift für Meteorologie" in 1897.

I'late I illustrates the different classes of typhoons enumerated in § 4. Figure 1 shows how the wind blows in a typhoon as determined by aid of observations made at the Hongkong Observatory during the years 1884-87 inclusive. Figure 2 shows a typhoon in the Formosa Channel on the 21st and 22nd August, 1884. The height of the barometer is noted near the isobars. The arrows fly with the wind, whose force is given in numbers. The large arrow shows the direction in which the whole disturb- ance was progressing. Figure 3 shows a typhoon that was lying a little N of Formosa on the 17th July, 1890. The number of feathers on the arrows indicates half the wind-force, e.g., 5 feathers means force 10. Figure 4 shows a typhoon in the China Sea on the 15th November, 1891. Figure 5 oue on the 20th November, 1891, that was moving towards SW. Figure 6 one on the 13th November, 1895. §L THE TYPHOONS IN THE EASTERN SEAS.

It appears that typhoons in the China Sea originate in elongated slight depressions, which some- times lie across the Philippines as well as the China Sea, but usually exist only over the sea extending sometimes far into the Pacific. To the north of them it blows moderate NE breezes and south of them somewhat less strongly from the SW. The NE breezes reach generally only as far as northero Formosa in summer, but in autumn the NE (and farther north the NW) monsoon blows much farther north. Sometimes the SW breezes to the south of the axis of the depression are stronger than the NE breezes to the north of it, and extend apparently down to the equator and are probably a continuation of the SE trade. To the E of these depressions in the Philippines there are light S and SE breezes. In Annam it probably blows from the N. In summer these depressions begin with rising pressure in the interior of China or in Japan. In autumn it seems the pressure rises slightly near the equator and SW winds extend gradually northward over the China Sea. In January and February depressions do not occur. During the rest of the year they occur about once a month on an average. During the summer months and in autumn they frequently give rise to a typhoon or a small circular depression. The trough-like depression then ceases to exist. In spring they do not alter into typhoons but cease to exist owing to the NE monsoon filling them and spreading to the southward.

The depressions have their major axes lying E and W, or ENE and WSW. Their average latitude from June to September is 17° N, later more southerly, and in November perhaps 10° N. They do not appear to move at all, and they may be traced for 3 or 4 days. The barometer is read little more than a tenth of an inch lower in the axis than along the coasts all round them. Along these coasts light winds circulate against the hands of a watch. In such depressions the weather is squally and wet, and the wind variable,-frequently in heavy squalls with great downpour of rain, but thunder is seldom heard. It appears that in such squalls S wind happens to extend itself northwards and N wind south- wards, and revolving storms are thereby generated. If this occurs in the middle of the China Sea, it is likely to give rise to a typhoon. Of course, it more often happens that a circular storm originates near the E and W end of the elongated depression as the winds there already revolve as in a rotary storm except to the W or E of the centre forming, so that the N or respectively S squalls need only gain ground on one side, but in such cases only minor circular depressions or very small typhoons are originated.

When the trough stretches from south of Hainan through the Bashee Chaunel right out into the Pacific to the south of Japan and the NE and SW winds on either side of it are fresh or strong, the conditions have often been mistaken for two typhoons, one in the China Sea and one to the south of Japan, before ever any typhoon was formed.

The heavy rain is, of course, not the cause of the phenomena, for the rain itself is caused by the air rising in the axis of these depressions, also the water-vapour condensing gives out heat and thus in the first instance makes the mercury rise in the barometer before a squall, but there cannot be any doubt that the quantity of water-vapour condensed to form perhaps 10 inches of rain per day, and whose pressure is thus abstracted from the barometric pressure of the air, causes the permanency of the depressions. It is different with the rainfall in the SW monsoon. That is spread over a large area and does not give rise to a low pressure in one spot surrounded by higher pressures.

It is rather difficult to say whether a depression in the China Sea, when its existence has been ascertained, is a typhoon or only a minor disturbance, but if the following signs are observed exactly as now to be explained, then it is certain to be a typhoon. A minor depression gives signs less well

marked and more confused.