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lion of the lower portion of the typhoon divided. The corresponding divisions are lettered inaccordance with these zones A, B, C, and D. (See PL XXXIX.)

Zone A —Typhoon distant.—If the needle has reached this division and the barometer showsan inclination to fall we may be tolerably certain that the center of a typhoon is between 500and 120 miles distant. The width of this zone is 4 millimeters (0.16 inch).

Zone B —Typhoon near.—The needle moving in this division indicates that most probably thereis a cyclonic vortex somewhere at a distance varying between 120 and 60 miles. The range ofpressure for this zone is, like that of the preceding, 4 millimeters (0.16 inch).

Zone C—Typhoon very near.—When the needle reaches this division, which has a range of7 millimeters (0.28 inch), it is a sign that the center of the typhoon has approached to within60-10 miles. A careful study of the minima recorded during the passage of more than 280cyclones makes it almost certain that the vortex will have arrived at less than 60 miles distancefrom Manila when the barometer has fallen to 747 millimeters (29.41 inches) at the time of thedaily minimum.

Zone D—Typhoon at the place of observation.—Distance of the center, 10-0 miles. Havingcarefully noted the barometric minima at the passage of typhoons over a place or very close to it,we have observed that in almost every instance the upper limit of atmospheric pressure is 740millimeters (29.134 inches). Toward the left, however, this division is undefined, though thebarometer very rarely falls below 700 millimeters (27.56 inches).

Before we conclude the description of this portion of the barocyclonometer we must call attentionto the following important points:

(a) The bearing of the center and the direction of its progressive movement can not be ascer-tained from the observations of the barometer alone; they must be determined by means of the lawsof cyclonic movements.

(&) Nor does the barometric height as such give us any clue as to the force of the wind orthe violence which the cyclone is likely to develop. The intensity of a cyclone depends in the mainon the barometric gradient, and the latter, in turn, is not dependent on the absolute amount ofpressure, but on the rapidity with which it diminishes, as is evident from the definition of barometricgradient and has been proven by experience. The inclination of the cyclone axis, which also has noconnection with the barometer level, has a very great influence upon the force of the wind.

(c) Once more we repeat, that the aneroid must be of the very best quality to be of service. Wehave found on board ships instruments of a very poor class, which are of no use whatever. To makesure of the quality of the aneroid it ought to be compared for some time with a standard barometer.But even after its excellence has thus been established before the apparatus was set up, this compar-ison should be repeated at least once a year, especially after the aneroid has been exposed to verygreat oscillations—e. g., in passing through the vortex of a cyclone. In this case the comparisonshould be made as soon as practicable. These investigations are best instituted at one of the largeobservatories of the Par East—e. g., Hongkong, Shanghai, Tokio, Manila—whose directors will, nodoubt, willingly lend their aid.

( d) It is very important to have a clear idea of what is meant by normal barometric height.The normal barometric height of a place is the mean of the barometer readings made at that placeduring a given period of time, reduced to sea level and to 0° C. (32° F.), also corrected, if onewishes, for the effect of gravity. Where the range of pressure is small, as is the case in the Tropics,it is convenient to group the months together, and, taking the mean of all the readings of each group,deduce therefrom the normal barometric level for the respective groups. Where, however, the annualvariation is considerable, it will be necessary to find from the respective monthly means the normalatmospheric pressure for each month.

The normal barometric height, therefore, varies (1) with the various seasons of the year, (2)with latitude, and (3) with the geodetic position. 1 Hence it is also easily understood that the

'By geodetic position we denote here the position of a place with regard to continents and seas, whether theplace is on the coast (and on which coast) or in the interior, on an island, how far from the continent, etc.

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