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Appendix B.
THE TYPHOONS IN 1892.
By W. Doberck and F. G. Figg.
It appears that typhoons in the China Sea originate in elongated slight depressions, which some- times but rarely lie across the Philippines as well as the China Sea, but usually exist only over the sea. 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 northern 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. 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 usually give rise to a typhoon or a small circular depression. The troughlike 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 16° 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 corner 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.
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 indications explained in the “Law of Storms in the Eastern Seas" (Hongkong 1886) are observed exactly as laid down in the pamphlet, then it is certain to be a typhoon. A minor depression gives signs less well marked and more confused.
When the wind rises in a typhoon it blows in gusts and the mercury heaves in the barometer. When the wind has reached force 11 it blows in fierce squalls of sometimes from 10 to 15 minutes duration, while the mercury heaves up and down as much as tenth of an inch. The mercury often gives a jump upwards as the wind begins to veer in a squall. Then it drops down and gives another jump upward as the wind comes back to nearly its old direction. During these squalls an enormous quantity of rain falls in a few minutes. The temperature falls and rises a fraction of a degree a more. The wind does not return to quite the former direction, except just in front of the centre. At the time when the centre is nearest, a fierce squall is usually felt and in that squall the direction of the wind changes considerably and the barometer begins to rise. The squalls appear to be caused by an up-and-down movement of the air. As the air comes rushing down, the raindrops tend to evaporate in the hotter stratum near the earth's surface and owing to the increased tension of water-vapour, the barometer (after a fall caused by the cold of evaporation) begins to rise. The wind veers towards the direction of the wind above, which latter is known from the motion of the clouds. Then the air starts to rise with a deluge of rain, caused by the condensation of vapour arriving at the cooler stratum above, while the barometer (after a rise caused by the heat of liquefaction) drops down owing to the cessation of the pressure of water-vapour condensed into the rain fallen, and the wind resumes the direction determined by the central depression; for the latter is so great in a typhoon and gradients so steep near the centre, that subsidiary depressions have never occurred in the China Sea.
Within 75 miles of the centre of a typhoon, or within 50 miles in case of a small typhoon, the angle between the direction towards which the wind is blowing and the direction in which the centre is