Page:Physical Geography of the Sea and its Meteorology.djvu/23



1.—This Plate combines in its construction the results of 1,159,353 separate observations on the force and direction of the wind, and a little upwards of 100,000 observations on the height of the barometer at sea. The wind observations embrace a period of eight hours each, or three during the twenty-four hours. Each one of the barometric observations expresses the mean height of the barometer for the day; therefore each one of the 100,000 may itself be the mean of many, or it may be only one. Suffice it to say, that 83,334 of them were obtained by Lieutenant Andrau from the logs of Dutch ships during their voyages to and fro between the parallels of 50° N. and 36° S.; that nearly 6,000 of them were made south of the parallel of 36° S., and obtained from the log-books at the Observatory in Washington; that for the others at sea I am indebted to the observations of Captain "Wilkes, of the Exploring Expedition, of Sir James Clark Ross, on board the Erebus and Terror, in high southern latitudes, and of Dr. Kane in the Arctic Ocean. Besides these, others made near the sea have been used, as those at Greenwich, St. Petersburg, Hobart Town, etc., making upwards of 100,000 in all. This profile shows how unequally the atmosphere is divided by the equator.

The arrows within the circle fly with the wind. They represent its mean, annual direction from each quarter, and by bands 5° of latitude in breadth, and according to actual observation at sea. They show by their length the annual duration of the wind in months. They are on a scale of one twentieth of an inch per month, except the half-bearded arrows, which are on a scale twice as great, or one tenth of an inch to the month. It will thus be perceived at a glance that the winds of the longest duration are the S.E. trades, between the parallels of 5° and 10° south, where the long-feathered arrows represent an annual average of ten months.

The most prevalent winds in each band are represented by full-feathered arrows; the next by half-feathered, except between the parallels of 30° and 35° N., where the N.E. and S.W. winds, and between the parallels of 35° and 40°, where the N.W. and S.W. winds contend for the mastery as to average annual duration.

The rows of arrows on each side of the axis, and nearest to it, are projected with the utmost care as to direction, and length or duration.

The feathered arrows in the shading around the circle represent the crossing at the calm belts, and the great equatorial and polar movements by upper and lower currents of air in its general system of atmospherical circulation.

The small featherless and curved arrows, n q r s, on the shading around the circle, are intended to suggest how the trade-winds, as they cross parallels of larger and larger circumference on their way to the equator, act as an undertow, and draw supplies of pure air down from the counter-current above; which supplies are required to satisfy the increasing demands of these winds: for, as they near the equator, they not only cross parallels of larger circumference, but, as actual observations show, they also greatly increase both their duration and