414 ' THE PHYSICAL GEOGRAPHY OF THE SEA. 



CHAPTER XIX. 



§ 781-808. — STORMS, HURRICANES, AND TYPHOONS. 



781. Plate V. is constructed from data furnished by the Pilot 

 Plate V. Charts, as far as they go, that are in process of con- 

 struction at the National Observatory. For the Pilot Charts, the 

 whole ocean is divided off into "fields" or districts of five degrees 

 square, ^. e., five degrees of latitude by five degrees of longitude, 

 as already explained on page xxxii., Plate Y. 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 tlae 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 

 5?/-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. hy E., W. hy S., etc. Moreover, any^attempt, for 

 the present, at greater nicety would be over-refinement, for nav- 

 igators 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 ves- 

 sel 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 diffi- 

 culty in understanding the wind diagram (Plate Y.), and in form- 

 ing 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 YIIT. As the compiler wades through log-book after log- 



