Plate VI illustrates the position of the channel of the Gulf Stream (Chap. II.) for summer and winter. The diagram A shows a thermometrical profile presented by cross-sections of the Gulf Stream, according to observations made by the hydrographical parties of the United States Coast Survey. The elements for this diagram were kindly furnished me by the superintendent of that work. They are from a paper on the Gulf Stream read by him before the American Association for the Advancement of Science at its meeting in Washington, 1854. Imagine 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 vessel.
The black lines x, y, 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 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 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 (§ 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 weather—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 XII. speak for themselves. They are orographic for the North Atlantic Ocean, and exhibit completely the present state of our knowledge with 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 from the commencement of the system to Dayman's soundings in the Bay of Biscay, 1859; 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.
