8

llegarding the use of the foregoing table we must add:

(1) The limiting months of two consecutive groups exhibit characteristics of both groups—that is, May and June, November and December, March and April, September and October.

(2) The same may be said of the parallels of latitude adjacent to the limits of above zones—that is, 10°—12°, 31°-33°, etc.

(3) The barometer readings have reference to the lowest reading occuring during the daj rand accordingly either to the morning or the afternoon minimum.

(4) Whenever the barometer at the time of the daily minimum falls as far as the tabularvalue corresponding to the place of observation and the time of the year, the observer may bereasonably sure of the presence of an atmospheric disturbance, but he will not be able to tell.whether the storm will burst over his location or not unless he is acquainted with the character-istic barometric movements of the region. It must be well borne in mind that in zone A thediurnal and nocturnal oscillations are not completely lost, but only modified. It is on thisknowledge that the observer has to found his prevision of the clanger into which he may be run-ning, as well as the probability of avoiding it.

For the convenience of the observer the data of the above table are also engraved on the lowerhalf of the movable ring surrounding the face of the aneroid. In the lowest section of the annularsector are found the months constituting the groups 1, 2, 3. To the right and left of it are given thedifferent zone limits and the corresponding barometer readings for the various groups—e. g.,

1 ) 2 ) 3 ) 1 ) 2 ) 3 )

0°-ll°, 29.76 (inches), 756 (millimeters), reads: “In the zone bounded by the equator and theeleventh parallel of north latitude the mean atmospheric pressure at the outskirts of a typhoon is29.76 inches (756 millimeters) throughout the year.”. In the zone included between the eleventhand seventeenth degrees of north latitude the same limit will he seen to be 29.76 inches (756 milli-meters) for the first group and 29.73 inches (755 millimeters) for groups 2 and 3, while for the-zone 25°-32° north latitude there is not only a different value assigned for each group but twodifferent values for group 2, one for April and May, the other for October and November. Thus itis seen that the whole of the table is given on the instrument.

The process of adaptation is best shown by an example. Suppose a captain finds that onMay 15, at a given instant, the ship’s place is 7 h 46.4 m east and 18° 3' north. Turning to hisaneroid, he finds 29.76 inches (756 millimeters) to be the mean upper limit of pressure indicatingstormy weather in his latitude and month (group 2). He will now turn the ring until the redarrow, above spoken of, points to 29.76 inches, and his barometer is adjusted as far as weatherindications are concerned. The farther the needle points to the right of the red index, so muchthe steadier the weather will be. But should, after adjustment, the needle either constantly orat least at the hours of minimum point to the left, the ship has entered the outermost zone of atyphoon.

The same arrangement serves also to correct the indications for elevation of the instrumentabove sea level, etc. The graduations corresponding to the heights of the mercurial column at0° C. (32° F.) and the level of the sea, any considerable elevation of the barometer above saidlevel can not be neglected. Unless great heights are in question we may assume that the decreaseof atmospheric pressure is roughly 0.01 inch for every 10 feet (or about 1 millimeter per 11meters) elevation. This amount is therefore to be subtracted from the tabular reading and theindex arrow set accordingly. On board ship the elevation can never give rise to great errors, butthe case may be very different with instruments installed on land. Unless the elevation is smallthe corresponding correction should be ascertained and applied. If this precaution were neglecteda barocyclonometer at an elevation of 800 feet might cause considerable alarm by indicating atyphoon in close proximity (10 to 60 miles), when it should promise settled fair weather.

(3) Third group—“Typhoon.”—This group corresponds to “stormy weather” of the ordinaryaneroid, because, the barometer being adjusted according to directions just given, the needle nevermoves into this section unless the instrument is under the influence of a typhoon less than 500 nauti-cal miles distant. This section, the largest of all, is subdivided so as to make its parts correspondto the atmospheric pressures prevalent in the four zones, into which we consider the horizontal sec-