GRANT) CURRENTS OP ATMOSPHERIC CIRCULATION. 
655 
direction of the Sun’s diurnal relative motion through the sky from its rising in the 
east to its setting in the west. He does not indicate any knowledge of the fact that 
on the two sides of the Equator in tropical regions there are two Trade-Wind zones, 
one on each side, in each of which the wind prevails from east to west, with an 
accompanying motion in each case towards the Equator. 
We may, indeed, suppose, that such knowledge was only gradually acquired, chiefly 
by mariners, and was but vaguely and imperfectly intercommunicated among them, 
and was spread very little among others during a long period of time. I do not 
suppose that any remarkable step in the discovery and promulgation of knowledge of 
the prevalent courses of the winds in those seas in and about the Torrid Zone is to be 
attributed to any one person in particular, nor that there was, indeed, any very 
important and clear promulgation of the floating knowledge on the subject until the 
time when the astronomer, Halley, collected and systematized a large amount of 
valuable information, and presented it to the Boyal Society, in his paper in the 
‘ Transactions ’ of 1686, to which I shall make particular reference a little further on. 
It may be well at the present stage, before going further into the history of 
speculations, to draw attention to the chief features of the Trade Winds and other 
perennially prevalent air currents, as they present themselves very manifestly to the 
notice of mariners. 
The mariners on board a ship at sea, it is to be observed, however, have direct 
cognizance only of the wind blowing at the spot on the ocean’s expanse where for the 
time being their ship is situated. They can make no observations on the winds 
blowing at the same moment 100 miles away, and the vault of the sky above them 
presents to their eyes no adequate indication of the upper currents, or of the places 
whence these come or whither they are going in their circuits. But even long ago, by 
the collation among navigators of facts contemporaneously observed by various seamen, 
important knowledge was acquired gradually as to the general character of contempo¬ 
raneously existing air currents at the surface of the sea, without the aid of any 
trustworthy theory as to the continuations of such currents in circulation through the 
upper regions of the atmosphere. It is further to be noticed that the geographical 
distribution of sea and land, presenting as it does great regions of ocean, and large 
continents themselves varied with mountain ranges and Jow-lying plains, introduces 
great local variations in the conditions determining the courses of winds, and prevents 
the institution of any complete uniformity in the character of the air currents all 
round the Equator, or throughout zones between any parallels of latitude. 
But in the Atlantic and Pacific Oceans there are extensive regions within, and 
adjacent to, the Torrid Zone, in which the winds blow with remarkable constancy 
from the east while converging also from north and south at the two sides, towards a 
medial belt of calms and rains which is situated along, or very near to, the Equator. 
These remarkably persistent winds blowing in the northern hemisphere from the 
north-east, and in the southern from the south-east, are called the Trade Winds. The 
