EXPLANATION OF THE PLATES. XXXi 



squares, tlie months of the year being the ordinates, and the points of the compass 

 being the abscissa. As the wind is reported by a vessel that passes through any 

 part of the parallelogram, so is it assumed to have been at that time all over the par- 

 allelogram. From such investigations as this the Pilot Charts are constructed. 



Plate VL 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 hydrograph- 

 ical parties of the United States Coast Survey. The elements for this diagram were 

 kindly furnished me by the superintendent of that w^ork. 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 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 x, y, s, in the Gulf Stream, show the course w^hicli those threads 

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

 unfortunate steamer San Francisco would take after her terrible disaster in Decem- 

 ber, 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 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. 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 dia 

 grams 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 (§ G30), 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 

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

 and the most favorite places of resort for the whale — right in cold, and sjjerm in 

 w^arm water — are also exhibited on this chart. 



Plate X. (p. 214) represents the curves of specific gravity and temperature of tin 

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



