xxiv 
EXPLANATION OF THE PLATES. 
Plate VIL 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- 
l)ort 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. Where 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 average 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 (J^ 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 
ito 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 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 elicited 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. 
