EXPLANi.TION OF THE PLATES. XXVU 



Capes of Virginia straight out to sea, crossing the Gulf Stream 'at right angles, and 

 taking the temperature of its waters at the surface and at various depths. The dia- 

 gram shows the elevation and depression of the thermometer across this section as 

 they were actually observed by such a vessel. 



The black lines r, y, z, in the Gulf Stream, show the course which those threads 

 of warm waters take (§ 130). The lines a, 6, show the computed drift route that 

 the unfortunate steamer " San Francisco " would take after her terrible disaster in 

 December, 1853. 



Plate VIT. 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 

 pi'ecipitation is small) are supposed to get their vapours from ; where, as surface 

 winds, they are supposed to condense portions of it ; and whither they are supposed 

 to transport the residue thereof through the upper regions, retaining it until they 

 again become surface winds. 



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

 parts of the ocean. 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 per- 

 pendicular 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 (§ 630), 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 north-west and the south-west, the idea intended 

 to be conveyed is, that the prevailing direction of the wind is between the north-west 

 and the south-west, 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 

 (§ 742). Farther researches will enable us to improve this chart. The sargasso seas 

 and the most favourite places of resort for the whale — right in cold, and sperm in 

 warm water— are also exhibited on this chart. 



Plate X. (p. 208) represents the curves of specific gravity and temperature of the 

 surface waters of the ocean, as observed by Captain John Rodgers in the U. S. ship 

 *' Vincennes" on a voyage from Behring's Strait via California and Cape Horn to 

 New York. 



Plates XI. and Xn. 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 XI. 



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 AVashington Observatory, 

 in 1855. 



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

 other vertical lines, months , - - 



days in a hundred. 



