38. Progressive Motion. In all cases within the tropics these storms commence in the east. For some days they travel slowly along a path not exactly west but inclining a point or two toward the pole of the hemisphere in which they begin; as they advance they seem to be more inclined to curve away from the equator, and when reaching the

twenty-fifth to thirtieth parallel of latitude they generally curve still more, at the same time increasing their progressive motion until they move in a northeast direction in the northern hemisphere and in a southeast direction in the southern hemisphere, as shown in Fig. 11.

But some cyclones seldom or never recurve; thus, the cyclones of the Bay of Bengal move, according to the season, from east to west or from south southeast to north northwest. The typhoons of the China Sea are of both types. Hurricanes also traverse the northern part of the North Atlantic Ocean from west southwest to east northeast without having recurved, as did the hurricane of October 20 to 23, 1897 (indicated in diagram of Fig. 12, by the curve numbered 21). Similarly, the progressive motion of cyclones in the Southern Pacific Ocean are sometimes east southeast and southeast, forming areas of low pressure, amalgamating as they pass onward, and then finally dispersing. As previously stated, the progressive motion is governed by local circumstances, but the rate of this motion has no necessary connection with the force of the wind.

The accompanying diagram, Fig. 12, compiled by the United States Hydrographic Office, shows the path followed by the center of each of the tropical cyclonic storms that have occurred in the North Atlantic Ocean during the 10-year period 1890 to 1899, the indicated points in each track marking the position of the storm center at Greenwich mean noon of successive days, the intervals between these points thus showing the distance traversed by the storm. center during approximately 24 hours. The dates of the several storms are as follows:

1. Aug. 27-Sept. 1, 1890 2. Aug. 19-Aug. 25, 1890 3. Aug. 19-Aug. 31, 1891 4. Sept. 4-Sept. 9, 1891 5. Sept. 16-Sept. 25, 1891 6. Sept. 28-Oct. 7, 1891 7. Aug. 17-Aug. 22, 1892 8. Aug. 15-Aug. 22, 1893 9. Aug. 23-Aug. 28, 1893 10. Sept. 6-Sept. 9, 1894 11. Sept. 20-Oct. 4, 1894 12. Oct. 5-Oct. 10, 1894 13. Oct. 12-Oct. 18, 1894

39. Shape, Diameter, and Barometric Pressure. The shape of a cyclone is seldom circular but rather oval or elliptic, and its diameter, which is generally very small at its origin, expands with the increase in latitude to as much as 800 or 1,000 miles. When the storm area is thus expanded the destructive character of the storm is lost, and it is then principally engaged in causing heavy rains in its vicinity. It is evident that the barometric pressure of a cyclone is least at its center, where it often is as low as 28 inches or less, and that the pressure is gradually increased toward the circumference of the storm area.

40. Direction of Winds.-The winds within a cyclone do not blow in perfect circles nor in straight lines but rather in irregular spirals. They blow in a more circular direction on the outsides of the cyclone than near its center. A cyclone, therefore, must not be thought of only as a rotating disk propelled mechanically from parallel to parallel, but also as an atmospheric eddy (see Fig. 13) that dies out in its rear but is rapidly renewed in its front.

41. Cyclone - Infested Regions.-Tropical cyclones are never met within the belt between 10° N and 10° S, but outside of these limits they occur in the following regions: North Atlantic Ocean-Western part, in the vicinity of the West Indies and along the southeast coast of the United States. South Atlantic Ocean-Southern part.

North Pacific Ocean-The China and Java seas and the coast of Japan.

South Pacific Ocean-Eastern part.

North Indian Ocean-Bay of Bengal and Arabian Sea. South Indian Ocean-Near Mauritius and Réunion islands. Cyclones are little known in the South Atlantic and South Pacific oceans. In the latter, they are but occasionally met with in the region east of Australia. In the northern gulfs of the Indian Ocean, particularly in the Bay of Bengal, cyclones occur with dreaded violence.

42. Cyclone Seasons.-Years of diligent observations have established the fact that the most cyclone-infested

regions of the globe are the West Indies, the Arabian Sea, and the China Sea. The worst cyclone months in the various regions are given as follows:

West Indies

September.

June to October, particularly August and

South Atlantic Ocean-December to February.

Bay of Bengal and Arabian Sea-April, May, October, and November.

South Indian Ocean-January to March.

China and Java seas-July to October.

43. Dangerous and Navigable Semicircles.-From the nature of cyclonic storms it is evident that the wind in the front and rear must be in a direction perpendicular to the line of progression a b, Fig. 13 (x), or nearly so; in other words, if the cyclone is moving in a north-northeasterly direction the wind in its front should be about east southeast and in its rear about west northwest. From this an important conclusion may be drawn, namely, that if we assume the area of the cyclone to be divided into two equal parts by the line of progression a b, and that another line ed is drawn through the center c perpendicular to a b, the front quadrant bed, in which the wind blows toward the line of progression, or track of center, is the most dangerous part of the cyclone, with the exception of the center itself. The rear quadrant a cd may also be considered as dangerous, because the direction of the wind will tend to carry the vessel that may happen to be there into the front quadrant and thence into the path of the center. These two quadrants, or the semicircle adb, are therefore known as the dangerous semicircle, and the other half aeb as the navigable semicircle, since the wind in the latter will blow away from in front of the storm center.

These two semicircles change sides when the hemisphere is changed, the dangerous semicircle always being to the right of the line of progression in northern latitudes and to the left in southern latitudes. The student will notice by examining Fig. 11 that in both cases the