326 THE PHYSICAL GEOGRAPHY OF THE SEA. 



CHAPTER XVII. 



STOEMS. 



Data for Plate V., $ 939.— T3'p]ioons, 936.— Monsoons in the China Sea, 937.— Mau- 

 ritius Hurricanes, 938. — Y/est India, ditto, 939. — Jansen on Hurricanes and Cy- 

 clones, 940. — Extra-tropic.il Gales, 950. — The Steamer San Francisco's Gale, 951.— 

 More Rains, Gales, &c., in the North than in the South Atlantic (Plate XIII.), 956. 



929. Plate V. is constructed from data furnished loj the Pilot 

 Charts, as far as thej 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 ex- 

 plained on page 23. Now, in getting out from the log-books ma- 

 terials for shov/ing, 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 dis- 

 trict ; and this is the only assumption that is permitted in the 

 whole course of investigation. 



930. 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 ^?/-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. bi/ 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 



