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standard value 760 millimeters is very improperly termed the general normal height of the barometerat the level of the sea, since the pressure at sea level is necessarily different at places differentlysituated, and even at one and the same place varies with the seasons.

From the preceding remarks we can draw the following conclusions, which are paramount inmeteorology:

(1) According to the very nature of normal barometric height, this furnishes us the only correctpoint of reference from which to estimate high and low barometers. For instance, for certain pointsof the interior of Siberia the monthly mean for January, corrected for elevation and temperature,gives as normal barometric height for this month 774 millimeters (30.472 inches). Consequently, atthese places the barometer has to be called low if at the time of the diurnal maximum it indicates apressure of 773 millimeters (30.433 inches), although this level is rather high if compared with theconventional value of 760 millimeters.

Whenever, therefore, the needle oscillates about the point of reference—that is, about the normalbarometric height of the place of observation for the season in which the reading is made—it is notpossible to say whether the weather will improve or become worse. The weather is variable becausethe normal barometric height of the place and time, and not 760 millimeters, is the barometer levelindicative of variable weather.

(2) Supposing, for example, that at a given point the normal barometric height is 769 millime-ters, it is perfectly absurd to say that this barometric level is 9 millimeters higher than the normalbarometric height at sea level for this place. The reason is that the idea of normal barometric heightessentially includes that the barometer means be reduced to the level of the sea.

II. The cyclonometer or wind disk.—The cyclonometer resembles the aneroid in shape anddimensions, but is, of course, an entirely different instrument. (See PI. XXXIX.)

The face plate of the cyclonometer, which is immovable, has a circular recess of a radius some-what less than the plate itself cut into its surface. On the rim thus formed are engraved the sixteenprincipal points of the compass. The whole face is covered by a glass plate, likewise immovable,which on its inner side has eight diameters intersecting at equal angles, etched in red. These linesare so placed that their extremities mark the sixteen points just mentioned, and consequently theyserve as lines of bearing all over the field of the wind disk. Underneath this cover glass, fitting intothe recess of the face plate, is a disk of silvered brass, which, by means of a knob passing throughthe cover glass, can be made to revolve around its center. Four concentric circles divide this diskinto five zones corresponding to the zones A, B, C, and P) and the central area of a typhoon. Aboveletters will be found in the respective annular sectors. . Across the central area is drawn a heavyblack arrow, to be set in the direction in which tire storm is supposed to move, while in each of thezones the winds prevailing in it are represented by smaller arrows. For the right and left part ofthe cyclone (with reference to the central arrow) only three wind directions are given in each zone;but for the front and rear five of them are indicated, since it is in these portions that the influenceof the progressive movement of the vortex upon the directions of the wind is chiefly felt. 'Flicmissing arrows can easily be interpolated. As is readily seen, the whole arrangement reproduces ahorizontal section of a cyclone near the surface of the earth.

Outside of the cover glass will be seen two needles whose length equals the diameter of the face.These pivot at the center and can be moved directly by hand. One of them bears a graduation, theinner two-thirds of one of its halves being divided into 100 equal parts, with the zero point at thecenter. In the subsequent description of its use we shall refer to it as the “graduated needle.” Theother carries,- at the point marking the end of the inner two-thirds of its half length, a pivot aroundwhich a smaller needle can be turned. We may call it the “double needle.”

This may suffice for a description of the barocyclonometer. Its application to the problems,the solution of which it is designed to facilitate, will be shown in the following chapter.

Remarks.—(1) The directions of the wind are inclined toward the center of the cyclone.

(2) The degree of convergence is not the same a.t various distances, nor is it the same onvarious sides at the same distance from the center.