Page:Physical Geography of the Sea and its Meteorology.djvu/439

Rh CHAPTER XIX. § 781-808.—STORMS, HURRICANES, AND TYPHOONS. 781. Plate V.—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 "fields " or districts of five degrees square, i. e. five degrees of latitude by five degrees of longitude, as already stated in the "" 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 the 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., etc. 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 upon mark, he at last finds that, under the