USE OF THE BAROCYCLONOMETER.
If an observer after consulting the given data and rules lias satisfied himself of theapproach of a cyclone, lie must (by turning the projecting knob) set the central arrow of thewind disk in the mean direction of the tracks followed by the cyclones of the region during therespective seasons. For this purpose the mean trajectories followed by the typhoons during thevarious months of the year have been laid down in the large map accompanying this book. The.instrument is now ready for approximately indicating the position and direction of the center.
The first approximation.—The observer must get accustomed to imagining himself placedaccording to the state of the barometer and the direction of the wind at a point of the concentricrings of the wind disk; this will in no small degree facilitate his calculation. In order to arriveat a first approximation as to the whereabouts of the storm, we must note which of the windarrows represented in zone A corresponds to the direction of the wind prevailing at the place ofobservation . 1 If none of the arrows actually drawn corresponds exactly to this wind direction,it will not be difficult to find the place of the ring where the arrow ought to lie. Having foundthe wind arrow, we must set one of the two large needles in such a way that the plain end of itpasses through the starting point of the arrow on the circle; the other end of the needle will thenpoint toward the direction in which the center lies.
If the barometer falls, even though slowly, or if there is a partial change in the barometricalcurve but the wind is still blowing from the same point or from a direction close to it, this is aproof that the position of the center has been determined with sufficient accuracy. In proportionas the barometer keeps on falling, the indications given by the wind direction will be"more reliable.
The second approximation.—If by a further fall of the barometer and by a partial change inthe daily range of pressure the barometrical reading already corresponds to the ring for zone B, wemust determine the prevailing direction of the wind and find out the wind arrow in ring B whichcorresponds to this direction of the wind. Next we must bring the needle to cut the wind arrow atits starting point, as before, then the other end points with greater certainty than before towardthe direction in which the center lies. If the wind veers we must repeat the operation. If thewind veers without a total change in the daily curve of the barometer taking place, we may takeit for granted that the center will not pass over the place of observation. The instrument showsthe observer with sufficient accuracy for practical purposes not only the direction in which thecenter lies but also—and this is for the sailor most important—the distance of the center and thedirection in which it is most probably moving.
The third approximation—The determination of the direction of the center.—If the barom-eter keeps on falling, the observer determines tire prevailing wind direction as before, searchesfor the corresponding wind arrow and, using now the double needle, brings its single end to cutthe wind arrow as before. The end to which the small needle is fastened shows the position ofthe center. If the wind continues to blow from the same point and the barometer keeps on fallingwe need not proceed further; the center is approaching the place of observation exactly from thedirection which the needle indicates. If, however, the wind direction in two or three hours changes,then we must repeat the observation with the graduated needle, leaving the double needle un-
'Wit-h regard to all the applications of the barocyclonometer it is important to remember that, in speaking ofwind directions, the direction of the prevailing wind is meant, not that of passing gusts and squalls the directionsof which are often misleading, especially near the outermost parts of the typhoon.
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