EXPLAi^ATION OF THE PLATES. 
Plate I. (p. 70) 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 which it was traveling when it arrived 
at the calm zone. 
Plates II. and III. (p. 207) are drawings of Brooke's Deep-sea Sounding Appa- 
ratus, for bringing up specimens of the bottom 438). 
Plate IV. (p. 230) 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 five degrees of longitude. These par- 
allelograms are then subdivided into a system of engraved squares, the rnonths of the 
year being the ordinates, and the points of the compass being the abscissae. 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 558) 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. This dia- 
gram shows the elevation and depression of the thermometer across this section as 
they w^ere 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 55). The lines a, b show the computed drift route that the 
unfortunate steamer San Francisco would take after her terrible disaster in December, 
1853. 
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