METEOROLOGY. 



Trade-winds. The first great effect on the 

 atmosphere of the unequal temperature of the dif- 

 ferent parts of the earth, and especially of the 

 steady decrease of heat from the equator to the 

 poles, is to produce the two grand currents which 

 we have already described : an upper-current from 

 the equator to the poles, and an under-current 

 from the poles to the equator. If the earth were 

 at rest, these currents would blow exactly north 

 and south, but the daily revolution of the globe 

 has an effect in altering their directions. The 

 equator, or thickest portion of the earth's body, 

 considered as a ball revolving on an axis, moves 

 with the greatest rapidity in the daily whirl ; any 

 place upon it is carried round at the rate of 

 upwards of 1000 miles an hour. But the belts on 

 each side of the equator being smaller in circum- 

 ference, any point on one of them is moved with 

 proportionably less rapidity. Thus the belt or 

 zone at 60 of latitude has only half the circum- 

 ference of the equatorial zone ; so that a place 

 upon it will move round only 500 miles an hour. 

 Now the atmosphere revolves along with the 

 earth, and every portion of it will have the same 

 velocity as the place on which it lies. But if 

 the equatorial air, with its high velocity, is carried 

 away in the upper-current to a place with a lower 

 velocity, the air will still persevere in its equatorial 

 speed, and will consequently outrun the speed of 

 the place upon which it has come, and be felt as 

 a strong wind in the direction of the rotation. 



A similar explanation serves to shew that the 

 under-currents from the poles to the equator will 

 not be due north and south, but will have, in addi- 

 tion, a direction towards the west. 



According to this simple theory, the prevailing 

 winds ought everywhere on the earth's surface to 

 be easterly : in the northern hemisphere, they 

 should blow from the north-east ; in the southern 

 hemisphere, from the south-east. And at sea 

 within the tropics, and to some distance beyond 

 them, this is in fact the case. The great polar 

 currents are there known by the name of the trade- 

 winds, because they are so constant that they can 

 be calculated on by navigators. The trade-winds 

 begin to be felt at about 30 of latitude on each 

 side of the equator. Ships entering upon this belt 

 begin to feel a steady easterly breeze, which con- 

 tinues, although with some variation, to within 2 

 of the equator. The two currents from north and 

 south meet about the equator, and completely 

 neutralise each other ; and their meeting forms a 

 belt varying from 150 to 550 miles in width, called 

 the region of calms and variables. There is no 

 steady wind in this region ; its atmosphere is 

 generally calm, having at certain seasons light 

 southerly winds, interrupted by fearful storms and 

 tornadoes. 



Beyond the region of the trade-winds, the pre- 

 vailing currents are in the opposite direction 

 in the northern hemisphere, for instance, from the 

 south-west. To account for this, we must suppose 

 that the two great currents change places, and 

 that about the latitude of 30 the upper equatorial 

 current descends to the surface of the earth. 



The region of the south-east trade-winds is much 

 larger than that of the north-east, the belt between 

 them lying generally not at the equator, but several 

 degrees north of it. The entire belt of the trades 

 shifts in some degree northwards in summer, and 

 southwards in winter, following the course of the sun. \ 



Direct proofs have been furnished that in the 

 regions of the trade-winds there are upper return- 

 currents flowing in, the contrary direction. The 

 ashes of the volcano of St Vincent, on one occasion 

 were carried from west to east, to the astonish- 

 ment of the inhabitants of Barbadoes, who were 

 experiencing at the time the easterly wind. 



Sea and Land Breezes. The great polar and 

 equatorial currents are modified by another class 

 of currents, of a more local character, arising from 

 the unequal susceptibility of the sea and the 

 land to the sun's heat. If the sun shine with the 

 same directness and force on two tracts of surface 

 the one water, and the other solid ground, the 

 ground will be most rapidly heated, and at the 

 end of the day it will be found much warmer than 

 the water. On the other hand, in the absence of 

 the sun, the land cools fastest, and at the end of 

 the night it will be found below the temperature 

 of the adjoining water. This difference causes an 

 inequality in the temperature of the columns of air 

 lying upon the two surfaces. When the land is 

 hottest, the air resting upon it will be hotter and 

 lighter than the air over the sea ; hence the cold 

 air from the sea will rush in upon the hot. Thus 

 during the day, when the land is warmer than the 

 sea, there will be a breeze from the sea to the 

 land, and during the night, a breeze from the land 

 to the sea. These sea and land breezes are felt 

 regularly on all the sea-coasts when not overborne 

 by other winds. 



Monsoons. Where a large tract of land joins a 

 great expanse of sea, as on the coasts of America 

 and the south of Asia, the sea and land breezes 

 become very considerable. The easterly trade- 

 winds are often reversed by them. But the most 

 powerful effect of the sea and land winds is annual, 

 or dependent on the seasons. The most remark- 

 able example of this is furnished by what are 

 called the monsoons from the Arabic word mau- 

 sim, a set time or season experienced along the 

 coasts of Africa, India, and China. When the sun 

 is far north in midsummer, and shines with direct 

 rays on the Asiatic peninsulas, the land in them is 

 rendered very much hotter, both day and night, 

 than the ocean, and a strong sea-wind sets in 

 :owards these coasts from the south-west that is, 

 in a direction opposite to the great current that 

 causes the trade-winds. Hence an immense wide- 

 spreading conflict arises, which begins about the 

 month of April, or two months before the sun 

 tias reached his extreme north declination. The 

 beginning of the conflict in April causes furious 

 rain, wind, and thunder-storms ; but in the course 

 of two or three weeks the sea-breeze completely 

 overcomes and suspends the trade-wind, and con- 

 tinues steadily to prevail from April to October ; 

 this is called the south-west monsoon ; and it can 

 be as much calculated on for navigation as the 

 trade-winds in their most uninterrupted regions. 

 The influence which sustains this wind becomes 

 gradually weaker as the sun moves southward, 

 and in October it becomes too weak to overcome 

 the trade-wind, and the two meeting with equal 

 force, cause a second violent tumult of rain and 

 storms, till in a short time the trade-wind prevails ; 

 which then continues during the winter half-year, 

 and forms what is called the north-east monsoon. 



In the Atlantic, a similar action is perceived. 

 In the summer-time, the land of North Africa is 

 made so much hotter than the adjoining sea, that 



