DESCRIPTION OF THE TYPHOON ENCOUNTERED IN THE CHINESE SEAS, BY H.I.R.M.'s FRIGATE NOVARA, ON THE 18TH AND 19TH AUGUST, 1858.
The path of the typhoon has been deduced from comparison with the readings of the barometer, with which it corresponds pretty accurately, if due allowance be made for the fact, that in determining it the various directions in which the line of centres runs must be calculated on the supposition that the orbit of the cyclone is circular, which it is not in reality, since at any considerable distance from the centre it must be elliptical. Hence it is apparent that the rate of velocity of the cyclone in advancing along its path follows no fixed law, whereas some such regularity undoubtedly exists among the masses of air encountered by the cyclone. Hence too the errors thus made in specifying the direction of the wind become of considerable importance in this connection, more especially in the event of the place of observation being at any distance from the centre, or that the path of the cyclone forms a sharp angle when wheeling round. Moreover, as actually experienced, the path of the typhoon would lie more near the line of the points of observation than a sketch founded upon such observations would indicate, and than a general comparison of the paths of cyclones founded upon the theory of their gyratory motion would substantiate, except in those cases where the observer has been directly in the path of the cyclone.
In our case the absolute distances, as specified in the annexed table (see p. 490) of fifteen different stations taken during the three days during which the cyclone and its premonitory and subsequent symptoms lasted, are only assumed, because simultaneous observations of the varying directions of the wind could not be taken at various points of the course of the cyclone, and in so far may be inaccurate, although the relative distances might possibly be tolerably correct.
The observations as to the direction of the wind at noon of the 18th August and at the ensuing midnight, give results contradictory to the
theory, since the wind in both cases is almost the same as would at midnight of the 19th indicate a central point, falling actually behind that portion of the path of the line of centres already traversed on the 18th. Upon this showing the direction of the wind at 6 P.M. of the 18th may be assumed as that of the centre of the cyclone. In fact, the path of the cyclone at this point lay parallel with the course the ship was holding, whence only trifling variations would be observable in the direction of the wind at those periods. Besides, the cyclone was at that time approaching the vertex of its orbit, in doing which it encountered the large and tolerably lofty island of Okinawa-Sima of the Loo-Choo group, which must have resulted in a certain expenditure of the force causing the gyratory movement of the cyclone. In analyzing the path of the cyclone, account must also be taken of the winds that prevailed from the 17th August up to midnight, although these are to be considered, with respect to the cyclone proper, only in so far as they were winds that had been altered in direction at the origin of the typhoon in conformity with the laws of cyclones, which by no means imply in all cases a perfect gyration. However, as these winds are varied in direction by the same causes which are in full activity in the case of the cyclones, such variations must follow the same laws, and the lines of centres which present themselves with reference to these as parts of a circular orbit, naturally lie in the same direction as that of the cyclone at its origin.
As early as the 13th August a marked alteration in the temperature of the air had been perceptible at Shanghai; the thermometer fell from between 86° and 95° Fahr. to between 73°.4 and 78°.8 Fahr.: easterly breezes set in, and the barometer rose in a remarkable manner for that latitude and season. On the 17th the weather was still fine, but the sun set red and fiery behind a dense mass of clouds.
The morning of the 18th broke with continued fine weather; but cumulous clouds were massed on the sky, and looked black and threatening to the N.E. By 8 A.M. the wind and sea had both risen materially. By 3 P.M. the roll of the sea was from N. by E., the sky became still more cloudy, and the clouds began to descend; banks of clouds in the direction of the central point. At midnight between the 18th and 19th altered course to W. by S., in order to run out of the cyclone by reaching its southern edge.
On the 19th at 8 A.M. a heavy sea from the northward, the sky a dense mass of clouds with very limited horizon; the whole aspect of the heavens a grey misty wrack of clouds, gradually falling lower and lower,—only in the direction of the central point was there visible a gloomy, leaden-coloured segment of clear horizon. From 4 P.M. to 8 P.M. the clouds completely enveloped us, so that it was barely possible to descry an object a cable's length from the ship; constant gusts of wind with fine rain or sea-spray; very heavy sea from the west, but the waves fairly decapitated by the wind as fast as they rose. By 11 P.M. a few dark clouds became visible in the S.S.E., and the horizon began to widen again.
20th. The sky still covered; in the west, white parallel bands of clouds, forming segments of circles: the masts and rigging covered with a crust of evaporated salt.