OBSERVATIONS on the PLATES

Belonging to LECTURE IX.

PLATE I. The upper figure illustrates the reasoning, page

182. If the whole fuperficies of the globe were ocean, and in confequence free from impediment or obstacle, it is probable, that the furrounding atmosphere would be so uniformly moved by the folar influence, that only one wind (and that following the courfe of the fun) would ever prevail throughout the globe.—Where the fun is vertical it is noon, and here the figure reprefents a very confiderable expanfion of the aerial fluid, which, as the globe turns (B) toward the fun, pushes forward thofe parts of the atmosphere toward which the fun advances, they increafing in heat continually, till the fun at length becomes vertical, while the parts (A) turning from the fun, become gradually cooler by his abfence, and the air rushes forward to supply the progreffive vacancy following the expanfion; this fcheme would represent the effects, were the fuperficies of the globe without land; but as it really is, the land checks and changes the course of the winds, fo that they are permanent only on fuch wide extended oceans as are beyond the influence of the land's attraction, as at C C C, and E E.

We obferve alfo, that the direction of the arrows (which fhews the course of the wind) CCC, is not the fame as those A and B; it gradually becomes affimilated to it, while at the distance D, the air feels little or nothing of these motions,

IX.

Rr

but

but is liable to variablenefs and changes from a variety of local and temporary caufes.

N. B. The greatest, expansion of the air is not actually where the fun is vertical, but fomewhat advanced into the afternoon, and may be confidered as at A, the heat increasing for fome time after the fun ceases to be vertical.

The under figure is a view of the globe, fhewing the courfes of the winds on each fide of the equator (EQ). AB is part of the great South Sea, the wind blowing steadily in a courfe combined of north and eaft toward the equator; but by the intervention of the lands, iflands, &c. between C and D, the wind is attracted out of its course, and deviates very confiderably from it. Paffing thefe parts, we observe that from E to F the wind blows fteadily again toward the equator; but it is from parts where there is no land, which being to the fouthward, its direction is now a combination of fouth and east, and in this course it blows till contradicted by the vicinity of land: which alters its direction, as at I. G fhews, that half the year the wind blows one way; H fhews, that the other half of the year it blows directly contrary. This effect takes place where the fea may be faid to be bounded by · projections of land; but is unknown where the occean is widely expanded: the fouthern as well as the northern parts of the globe fuftaining lefs heat, exhibit no fuch regular or periodical winds, but their cooler atmospheres are more actuated by local and particular caufes and circumftances.

PLATE