xxiv EXPLANATION OF THE PLATES. 



Plate VII. is intended to show how the winds may become geological agents. It 

 shows where the winds that, in the general system of atmospherical circulation, blow 

 over the deserts and thirsty lands in Asia and Africa (where the annual amount of 

 precipitation is small;, are supposed to get their vapors from ; where, as surface winds, 

 they are supposed to condense portions of it ; and whither they are supposed to trans- 

 port the residue thereof through the upper regions, retaining it until they again be- 

 come surface winds. 



Plate VIII. shows the prevailing direction of the wind during the year in all parts 

 of the ocean, as derived from the series of investigations illustrated on Plate VII. It 

 also shows the principal routes across the seas to various places. VV^here the cross- 

 lines representing the yards are oblique to the keel of the vessel, they indicate that 

 the winds are, for the most part, ahead ; when perpendicular or square, that the winds 

 are, for the most part, fair. The figures on or near the diagrams representing the 

 vessels show the averagie length of the passage in days. 



The arrows denote the prevailing direction of the wind ; they are supposed to fly 

 with it ; so that the wind is going as the arrows point. The half-bearded and half- 

 feathered arrows represent monsoons (^ 462), and the stippled or shaded belts the 

 calm zones. 



In the regions on the polar side of the calms of Capricorn and of Cancer, where 

 the arrows are flying both from the northwest and the southwest, the idea intended 

 to be conveyed is, that the prevailing direction of the wind is between the northwest 

 and the southwest, and that their frequency is from these two quarters in proportion 

 to the number of arrows. 



Plate IX. is intended to show the present state of our knowledge with regard to 

 the drift of the ocean, or, more properly, with regard to the great flow of polar and 

 equatorial waters, and their channels of circulation as indicated by the thermometer 

 (^ 528). Further researches will enable us to improve this chart. The most favorite 

 places of resort for the whale — right in cold, and sperm in warm water — are also ex- 

 hibited on this chart. 



Plate X. exhibits the actual path of a storm, which is a type (<5> 72) of the West 

 India hurricanes. Mr. Redfield, Colonel Reid, and others, have traced out the paths 

 of a number of such storms. All of this class appear to make for the Gulf Stream ; 

 after reaching it, they turn about and follow it in their course {^ 75). 



Mr. Piddington, of Calcutta, has made the East India hurricanes, which are similar 

 to these, the object of special, patient, and laborious investigation. He calls them 

 cycloins, and has ehcited much valuable information concerning them, which may be 

 found embraced in his " Sailor's Horn-book," " Conversations about Hurricanes," and 

 numerous papers published from time to time in the Journal of the Asiatic Society. 



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 the sea ; Plate XII. exhibiting 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 XI. 



