STORMS. 257 
CHAPTER XVI. 
STORMS. 
Typhoons, § 559. — Cyclones, 561. — West India Hurricanes, 563. — Extra-tropical 
Gales, 563. — The San Francisco's Gale, 564. — These Gales seldom occur at cer- 
tain Seasons, 565. — Most prevalent Quarter for the Gales beyond the Calm Belt of 
Capricorn, 566. — Storm and Rain Charts, 567 257 
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 oif 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 show- 
ing, in every district of the ocean, and for every month, how nav- 
igators 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 direc- 
tion 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 6y-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., &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 th-e vessel may be moving through the water, and 
the angle which her course makes with the true direction of the 
R 
