440 PHYSICAL GEOGRAPHY. 



day after day, by a permanent breeze, farther from their native shores, and inferred the 

 impossibility of returning, as they observed no change in its direction. Fortunately for 

 his fame, and for the world, the great navigator firmly held on his course, reached the 

 bounds of the before supposed illimitable ocean, and re-crossed it in the region of the 

 variables, to the north of the northern trade wind. Now, in passing from the Canaries 

 to Cumana, on the north coast of South America, it is scarcely ever necessary to touch 

 the sails of a ship ; and with equal facility the passage is made across the Pacific, from 

 Acapulco, on the west coast of Mexico, to the Philippine Islands. If a channel were cut 

 through the isthmus of Panama, the voyage to China would be remarkably facilitated 

 by the trade winds of the Atlantic and Pacific Oceans, be more speedy, agreeable, and 

 safe, than the usual route by the Cape, the chief interruption to its uniformity 

 occurring in the Caribbean Sea and the Gulf of Mexico, where the trade wind blows 

 impetuously, the sea is stormy, and the sky grey and cloudy. 



The theory respecting the origin of the trade winds, adopted by Dr. Dalton, Professor 

 Daniell, and Sir John Herschell, was first proposed by George Hadley, the brother of the 

 inventor of the quadrant, and embodies features of the previous theories of Halley and 

 Galileo, who both grappled with this great geographical phenomenon. It is founded 

 upon the rarefaction of the atmosphere of the torrid zone by the powerful heat to which 

 that region is subject, in connection with the different velocities of the earth's surface, 

 in different degrees of latitude, in the diurnal rotation. Heat rarefies and expands a 

 volume of air in a ratio equivalent to an addition of about seventy feet to the ordinary 

 height of the atmosphere for every degree of thermometrical measurement. As the sun 

 is always vertical at some place within the tropics, the average temperature of the earth's 

 surface in that region, bounded by the parallels of 23^ on each side of the equator, is 

 much higher than in latitudes to the north and south ; and the incumbent air acquiring 

 this higher temperature, is thereby rarefied and expanded. The consequence is, that 

 in obedience to hydrostatical laws, masses of air are continually buoyed up from the 

 surface, or swelled round the torrid zone in the form of a protuberant belt, the upper 

 strata flowing over, and running off in streams north and south towards the poles, where, 

 having been cooled and condensed, they descend, and flow over the surface towards the 

 equator, pouring in a perpetual current of air to supply the place of that buoyed up by 

 the heat of the tropics. Thus, there is a constant current in the higher regions of the 

 atmosphere, proceeding from the equator northward and southward to the poles ; and if 

 the earth were at rest, there would be a constant wind in the lower regions of the 

 atmosphere blowing directly from the poles to the equator, while in equatorial regions 

 the two streamlets would meet and neutralise each other's influence. But the earth is 

 not at rest ! It is incessantly whirling upon its axis, the surface moving at a rate which 

 varies according to the extent of the circumference. The velocity at the equator, where 

 the circumference is the greatest, is about sixteen miles a minute ; at 30 of latitude, 

 which is below the most southerly point of Europe, it is about fourteen miles in the same 

 time; and at 45, or about the centre of France, it is about eleven miles. As the 

 distance from the equator increases, north and south, the rate of the rotation thus becomes 

 less, because the circle of the earth's circumference diminishes in extent. Now a current 

 of air flowing from the north or south polar regions, and setting towards the equator, 

 will encounter as it proceeds an increased rotatory motion eastward, the direction of the 

 earth's axical revolution, and, not acquiring the new velocity at once, it will be left 

 behind, and seem to deflect towards the west just in proportion as it does not keep up 

 with the earth to the east. Hence, what would simply be a north or south wind but for 

 the earth's rotatory motion, becomes a north-east and south-east wind as it approaches 

 those regions where, the velocity of the globe being so much greater than where it 



