GRAND CURRENTS OP ATMOSPHERIC CIRCULATION. 
661 
Winds of the equatorial regions on both sides of the Equator. His theory further 
extends to explain the cause of the prevalence of winds from west to east in latitudes 
higher than those of the winds of equatorial regions, to which, except in the nomen¬ 
clature of Hadley himself, the name Trade Winds has been usually restricted; and 
this part of his theory may be represented as follows :— 
The equatorial surface of the Earth has a velocity of diurnal revolution from west 
to ea.st of about 1000 miles per hour. The air of the land and sea at and near the 
Equator participates nearly in the same velocity. The ascending equatorial belt of 
heated air retains as it ascends an absolute velocity from west to east nearly the same 
as that of the equatorial surface of the Earth. He supposes, then, in his theory, that 
the air floating out from the upper part of the rising belt to north and south over the 
equatorial zones of Trade Winds, and thence, still in the upper parts of the atmosphere, 
spreading over extensive regions of land and sea in latitudes higher than those of the 
Trade Winds, will, on reaching those regions whose velocities of diurnal revolution are 
much slower, be rushing forward from west to east quicker than do the portions of the 
Earth’s surface over which it successively arrives in floating poleward ; that greater 
speed of eastward motion of the air than of the Earth beneath being, however (as he 
indicates with a fair approach to clearness), kept in moderation by influences from the 
surface of the land or sea offering resistance to relative motions of the air above it. 
Further, he supposes that this upper air, while moving eastward quicker than does 
the Earth below it, gradually loses a great part of its previously acquired heat, and 
becomes less buoyant, and consequently descends gradually towards the surface of the 
Earth, the supply above being always maintained by fresh arrivals from the equatorial 
regions ; and he supposes that the descending air brings from aloft perpetually new 
supplies of west-to-east motion relative to the surface, and so maintains winds blowdng 
over the surface from west to east. The air then, after its descent from the sky 
towards the surface throughout extensive regions, must, I think, necessarily, under his 
theory—although he does not explicitly mention this—be supposed to flow T gradually 
back in the lower levels of the atmosphere towards the Equator, while also blowing 
prevalently from west to east, till it reaches again the outer border of the trade-wind 
region, thence to go forward repeating such a circulation as has just been described. 
Hadley concludes his paper with a short passage which, considered in reference to 
the crude condition of progressive opinions prevalent in respect to atmospheric circu¬ 
lation up to the time of the promulgation of his theory, is to be regarded as suggesting, 
though in somewhat vague and not entirely correct expression, a very notable and 
important principle. 
The passage is as follows :—“ That the N.E. and S.E. Winds within the Tropicks 
must be compensated by as much N.W. and S.W. in other Parts, and generally all 
Winds from any one Quarter must be compensated by a contrary wind somewhere or 
other ; otherwise some Change must be produced in the Motion of the Earth round its 
Axis.” 
