EXPLANATION Ox THE PLATES. XIX 



Plate YI illustrates the position of the channel of the Gulf Stroam (Chap. II.) 

 for summer and winter. The diagram A shows a therniometrical profile pre- 

 sented bv cross-sections of the Gulf Stream, according to observations made by 

 the hvdro-raphical parties of the United States Coast Survey. I ne elements for 

 ♦his dia'T^am were kindly furnished me by the superintendent of that work. 

 Thev arS from a paper on the Gulf Stream, read by him before the American 

 Association for the Advancement of Science, at its meeting in Washington, 18oJ. 

 Ima-ine a vessel to sail from the 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 diagram shows the elevation and depression 

 of the thermometer across this section as they were actually observed by such a 



^^The black lines x, v, z, in the Gulf Stream, show the course which those threads 

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

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

 December, 1853. 



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 vapours from ; where, 

 as surface winds, they are supposed to condense portions of it ; and whitherttiey 

 are supposed to transport the residue thereof through the upper regions, retaining 

 it until they again become surface winds. 



Pl.\te VIII. showst he 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 ot 

 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. 1 he figures on 

 or near the diagrams representing the vessels show the average length ot the 

 passage in days. , , , , 



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

 fly ivith it ; so that the wind is going as the arrows point. The half-bearded ana 

 half-feathered arrows represent monsoons (§ 630), and the stippled or shaded 

 belts the calm zones. _ . , ^ r. v 



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 prevailng 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 ther- 

 mometer (§ 742). Farther researches will enable us to improve this chart. Ihe 

 sargasso seas and the most favourite places of resort for the whale— rifflit in cold, 

 and sperm in warm weather— are also exhibted 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 Kodgers in the 

 U.S. ship 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 witn 

 regard 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 troin the 

 commencement of the system to Dayman's soundings in the Bay of Biscay, icJJ, 

 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 lucatan, Cuba, ban 

 Domingo, and the Cape de Verds, to the coast of Africa, marked A on i late Ai. 



