Physical Geography Of The Sea 1855/16

'''Typhoons, § 559. — Cyclones, 561. — West India Hurricanes, 562. — Extra-tropical Gales, 563. — The San Francisco’s Gale, 564. — These Gales seldom occur at certain Seasons, 565. — Most prevalent Quarter for the Gales beyond the Calm Belt of Caprieorn, 566. — Storm and Rain Charts, 567. '''

STORMS. 558. PLATE V. is constructed from data furnished by the Pilot Charts, as far as they go, that are in process of construction at the National Observatory. For the Pilot Charts, the whole ocean is divided off into districts of five degrees square, i.e., five degrees of latitude by five degrees of longitude, as already explained on page 23. Now, in getting out from the log-books materials for showing, in every district of the ocean, and for every month, how navigators have found the winds to blow, it has been assumed that, in whatever part of one of these districts a navigator may be when he records the direction of the wind in his log, from that direction the wind was blowing at that time all over that district; and this is the only assumption that is permitted in the whole course of investigation. Now if the navigator will draw, or imagine to be drawn in any such district, twelve vertical columns for the twelve months, and then sixteen horizontal lines through the same for the sixteen points of the compass, i.e., for N., N.N.E., N.E., E.N.E., and so on, omitting the by-points, he will have before him a picture of the “Investigating Chart,” out of which the “Pilot Charts” are constructed. In this case, the alternate points of the compass only are used; because, when sailing free, the direction of the wind is seldom given for such points as N. by E., W. by S., &c. Moreover, any attempt, for the present, at greater nicety, would be over-refinement; for navigators do not always make allowance for the aberration of the wind; in other words, they do not allow for the apparent change in the direction of the wind caused by the rate at which the vessel may be moving through the water, and the angle which her course makes with the true direction of the wind. Bearing this explanation in mind, the intelligent navigator will have no difficulty in understanding the wind diagram (Plate V.), and in forming a correct opinion as to the degree of credit due to the fidelity with which the prevailing winds of the year are represented on Plate VIII. As the compiler wades through log-book after log-book, and scores down in column after column, and upon line after line, mark after mark, he at last finds that, under the month and from the course upon which he is about to make an entry, he has already made four marks or scores, thus (1111). The one that he has now to enter will make the fifth, and he “scores and tallies,” and so on until all the abstracts relating to that part of the ocean upon which he is at work have been gone over, and his materials exhausted. These “fives and tallies” are exhibited on Plate V. Now, with this explanation, it will be seen that in the district marked A (Plate V.) there have been examined the logs of vessels that, giving the direction of the wind for every eight hours, have altogether spent days enough to enable me to record the calms and the prevailing direction of the winds for eight hours, 2,144 times of these, 285 were for the month of September; and of these 285 observations for September, the wind is reported as prevailing for as much as eight hours at a time: from N.,3 times; from N.N.E.,1; N.E.,2; E.N.E.,1; E.,0; E.S.E.,1; S.E.,4; S.S.E.,2; S.,25; S.S.W.,45; S.W.,93; W.S.W.,24; W.,47; W.N.W.,17; N.W.,15; N.N.W.1; Calms (the little o’s), 5; total, 285 for this month in this district. The number expressed in figures denotes the whole number of observations of calms and winds together that are recorded for each month and district. In C, the wind in May sets one third of the time from west. But in A, which is between the same parallels, the favorite quarter for the same month is from S. to S.W., the wind setting one third of the time from that quarter, and only 10 out of 221 times from the west; or, on the average, it blows from the west only 1 1/3 days during the month of May. In B, notice the great “Sun Swing” of the winds in September, indicating that the change from summer to winter, in that region, is sudden and violent; from winter to summer, gentle and gradual. In some districts of the ocean, more than a thousand observations have been discussed for a single month, whereas, with regard to others, not a single record is to be found in any of the numerous log-books at the National Observatory. 559. TYPHOONS. — The China Seas are celebrated for their furious gales of wind, known among seamen as typhoons and white squalls. These seas are included on the plate (VIII.) as within the region of the monsoons of the Indian Ocean. But the monsoons of the China Seas are not five-month monsoons (§ 475); they do not prevail from the west of south for more than two or three months. 560. Plate V. exhibits the monsoons very clearly in a part of this sea. In the square between 15º and 20º north, 110º and 115º east, there appears to be a system of three monsoons; that is, from northeast in October, November, December, and January; from east in March and April, changing in May; from the southward in June, July, and August, and changing in September. The great disturber of the atmospheric equilibrium is situated among the arid plains of Asia; their influence extends to the China Seas, and about the changes of the monsoons these awful gales are experienced. 561. In like manner, the Mauritius hurricanes, or the cyclones of the Indian Ocean, occur during the unsettled state of the atmospheric equilibrium which takes place at that debatable period during the contest between the trade-wind force and the monsoon force (§ 477), and which debatable period occurs at the changing of the monsoon, and before either force has completely gained or lost the ascendency. At this period of the year, the winds, breaking loose from their controlling forces, seem to rage with a fury that would break up the very fountains of the deep. 562. So, too, with the West India hurricanes of the Atlantic. These winds are most apt to occur during the months of August and September. There is, therefore, this remarkable difference between these gales and those of the East Indies: the latter occur about the changing of the monsoons, the former during their height. In August and September, the southwest monsoons of Africa (§ 479) and the southeast monsoons of the West Indies (§ 474) are at their height; the agent of one drawing the northeast trade winds from the Atlantic into the interior of New Mexico and Texas, the agent of the other drawing them into the interior of Africa. Its two forces, pulling in opposite directions, assist now and then to disturb the atmospheric equilibrium to such an extent that the most powerful revulsions in the air are required to restore it. 563. EXTRA-TROPICAL GALES. — In the extra-tropical regions of each hemisphere furious gales of wind also occur. One of these, remarkable for its violent effects, was encountered on the 24th of December, 1853, about three hundred miles from Sandy Hook, latitude 39º north, longitude 70º west, by the San Francisco, steam-ship (§ 72). That ship was made a complete wreck in a few moments, and she was abandoned by the survivors, after incredible hardships, exertions, and sufferings. Some months after this disaster, I received by the California mail the abstract log of the fine clipper ship EAGLE WING (Ebenezer H. Linnell), from Boston to San Francisco. She encountered the ill-fated steamer’s gale, and thus describes it:-- 564. “DECEMBER 24TH, 1853. Latitude 39º 15’ north, longitude 62º 32’ west. First part threatening weather; shortened sail: at 4 P. M. close reefed the top-sails and furled the courses. At 8 P.M. took in fore and mizen top-sails; hove to under close-reefed main top-sail and spencer, the ship lying with her lee rail under water, nearly on her beam-ends. At 1:30 A.M. the fore and main top gallant-masts went over the side, it blowing a perfect hurricane. At 8 A.M., moderated; a sea took away jib-boom and bowspritcap. In my thirty-one years’ experience at sea, I have never seen a typhoon or hurricane so severe. Lost two men overboard, saved one. Stove sky-light, broke my barometer, &c., &c.” 565. Severe gales in this part of the Atlantic — i.e., on the polar side of the calm belt of Cancer — rarely occur during the months of June, July, August, and September. This appears to be the time when the fiends of the storm are most busily at work in the West Indies. Daring the remainder of the year, these extra-tropical gales, for the most part, come from the northwest. But the winter is the most famous season for these gales. That is the time when the Gulf Stream has brought the heat of summer and placed it (§ 71) in closest proximity to the extremest cold of the north. And there would, therefore, it would seem, be a conflict between these extremes; consequently, great disturbances in the air, and a violent rush from the cold to the warm. 566. In like manner, the gales that most prevail in the extra tropics of the southern hemisphere come from the pole and the west, i.e., southwest. 567. Storm and Rain Charts for the Atlantic Ocean have already been published by the Observatory, and others for the whole seas are in process of construction. The object of such charts is to show the directions and relative frequency of gales in all parts of the sea, the relative frequency of calms, fogs, rain, thunder, and lightning. These charts are very instructive.