ASTRONOMY AND METEOROLOGY. 399 



ATMOSPHERIC MOVEMENTS. BY DANIEL VAUGHAN. 



The remarkable uniformity in the course of the winds, throughout the 

 greater part of the torrid zone, was ascribed by Dr. Halley to the effect of 

 solar heat combined with the earth's diurnal motion. The air investing the 

 equatorial regions, being rarefied by the excessive temperature which the sun 

 imparts, is compelled to yield its place to the more cool and dense bodies of 

 air which press from opposite sides along the surface of the land and water. 

 Aerial currents are thus caused to flow from northern and southern localities 

 to the equator, while others return along the upper atmospheric domain. In 

 approaching the equator, they must pass over a part of the earth's surface 

 having a more rapid motion than they could have acquired in the regions 

 from which they came; and as they fail to partake of the increased velocity, 

 an apparent westwaixl deflection is the consequence ; so that they become 

 the north-east and south-east trade winds which prevail over a large portion 

 of the tropical i-egions. But though correct in its main features, this theory 

 fails to show why the trade winds do not extend beyond the thirtieth par- 

 allels of latitude, and that they are confined within a narrower limit on the 

 Pacific Ocean, where the disturbances and obstructions which might arise 

 from the presence of land, are almost entirely removed. The efficiency of 

 the motive power which acts on the air, is proportional to the increase of 

 mean temperature for a given diminution of latitude ; and it is much greater 

 in the temperate than in the torrid zone. It would, therefore, seem that the 

 trade winds of the tropical oceans should prevail through a more extensive 

 range, or that each of the temperate zones should have an independent cir- 

 culation of regular winds, blowing from the north-east in our climates, and 

 from the south-east in the southern hemisphere. 



The want of a general and uniform atmospheric circulation in extra- 

 tropical regions, must be ascribed to the motion of the earth around its axis. 

 The centrifugal force arising from the rotation, not only keeps our planet 

 expanded at the equator, and maintains a suitable covering of air and water 

 between the tropics, but also lays some restrictions on the removal of matter 

 to different localities. The principle on which this result depends, will be 

 understood from a few considerations. Were the earth's diurnal motion sud- 

 denly reduced four per cent., there would be a diminution of eight per cent, 

 in the centrifugal force of its parts ; their equilibrium would require a dif- 

 ference of only twenty-four miles between equatorial and polar diameters, 

 and much of the waters would remove from their tropical abodes to the 

 vicinity of the poles. The air would also experience a similar movement, 

 and it would rush northward with greater impetuosity, if the water were 

 absent from our planet, or could be prevented from filling up the circum- 

 polar basins. Now the strict uniformity which the earth exhibits in its rota- 

 tion, cannot be preserved by every portion of its atmosphere. If a large body 

 of air, situated between the 44th and 46th parallels of north latitude, were 

 moving due westward at the rate of twenty-eight miles an hour, its actual 

 velocity of rotation about the terrestrial axis must be about four per cent, 

 less than that of the land and water over which it passed; and so great 

 would be the reduction in its centrifugal force, that it must press, to wards 

 the polar regions in the same manner that it would descend down an inclined 

 plane with a fall of nine inches in every mile. Were the movement of the 

 mass of air in an eastward direction, it would have an increased velocity of 



