EXPLANATION OF THE PLATES. 



Plate I. (p. 75) is a diagram to illustrate the circulation of the atmosphere (Chap. 

 III.). The arrows and bands within the circumference of the circle are intended to 

 show the calm belts, and prevailing direction of the wind on each section of those 

 belts. The arrows exterior to the periphery of the circle— which is a section of the 

 earth supposed to be made in the plane of the meridian — are intended to show the 

 direction of the upper and lower strata of winds in the general system of atmospher- 

 ical circulation ; and also to illustrate how the air brought by each stratum to the 

 calm belts there ascends or descends, as the case may be ; and then, continuing to 

 flow on, how it crosses over in the direction in v/hich it was traveling when it arrived 

 at the calm zone. 



Plates II. and III. (p. 250) are drawings of Brooke's Deep-sea Sounding Appa- 

 ratus, for bringing up specimens of the bottom [^ 701). 



Plate IV. (p. 293) is intended to illustrate the extreme movements of the isotherms 

 50°, 60°, 70°, &c., in the Atlantic Ocean during the year. The connection between 

 the law of this motion and the climates of the sea is exceedingly interesting. 



Plate V. is a section taken from one of the manuscript charts at the Observatory. 

 It illustrates the method adopted there for co-ordinating for the Pilot Charts the winds 

 as reported in the abstract logs. For this purpose the ocean is divided into conven- 

 ient sections, usually five degrees of latitude by live degrees of longitude. These par- 

 allelograms are then subdivided into a system of engraved squares, the months of the 

 year being the ordinates, and the points of the compass being the abscissse. As the 

 wind is reported by a vessel that passes through any part of the parallelogram, so it 

 is assumed to have been at that time all over the parallelogram. From such investi- 

 gations as this the Pilot Charts (<$» 929) are constructed. 



Plate VI. illustrates the position of the channel of the Gulf Stream (Chap. I.) 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 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 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, z, in the Gulf Stream, show the course which those threads 



of warm waters take (<^ 57). The lines a, h 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 



