xxxii EXPLANATION OF THE PLATES. 



Vincennes, on a voyage from Behring's Strait via California and Cape Horn to New 

 York. 



Plates XI. and XII. speak for themselves. They are orographic for the North 

 Atlantic Ocean, and exhibit completely the present state of our knowledge with re- 

 gard to the elevations and depressions in the bed of that sea as derived from the deep- 

 sea soundings taken by the American and English navies from the commencement 

 of the system to Dayman's soundings in the Bay of Biscay, 1859 ; Plate XII. ex- 

 hibiting a vertical section of the Atlantic, and showing the contrasts of its bottom with 

 the sea-level in a line from Mexico across Yucatan, Cuba, San Domingo, and the 

 Cape de Verds, to the coast of Africa, marked A on Plate XL 



Plate XIII. — The data for this Plate are furnished by Maury's Storm and Rain 

 Charts, including observations for 107,277 days in the North Atlantic, and 158,025 

 in the South ; collated by Lieutenant J. J. Guthrie, at the Washington Observatory, 

 in 1855. 



The heavy vertical lines, 5°, 10°, 15°, etc., represent parallels of latitude ; the o'ther 

 vertical lines, months ; and the horizontal lines, per cents., or the number of days in 

 a hundred. 



The continuous curve line stands for phenomena in the North, and the broken 

 curve line for phenomena in the South Atlantic. Thus the Gales' Curs^e shows that 

 in every hundred days, and on the average, in the month of January of different 

 years, there have been observed, in the northern hemisphere, 36 gales (36 per cent.) 

 between the parallels of 50° and 55° ; whereas during the same time and between 

 the same parallels in the southern hemisphere, only 10 gales on the average (10 per 

 cent.) have been reported. 



The fact is here developed that the atmosphere is in a more unstable condition in 

 the North than in the South Atlantic ; that we have more calms, more rains, more 

 fogs, more gales, and more thunder in the northern than in the southern hemisphere, 

 particularly between the equator and the 55th parallel. Beyond that the influence 

 of Cape Horn becomes manifest. 



Plate XIV. (§ 838) shows the limits of the unexplored area about the south pole. 



Plate XV. shows the average number of days annually, and for every degree of 

 latitude between the parallels of 60° north and 60° south that the winds have north- 

 ing in them on one hand, and southing in them on the other ; also, the proportion 

 of calm days, vide p. 156. 



