SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 457 



After having shown, as I have previously done, that the same laws 

 that govern the movements of water in general also govern the move- 

 ments of currents of air, and, after having shown in this i)aper, that the 

 same laws that govern the movements of water in canals and free cur- 

 rents in the sea also govern the movements of the largo masses of air 

 which form hurricanes, I consider it almost as a necessary consequence 

 that currents of water in the sea and currents of air in the atmosphere 

 are subject to the same natural laws. Basing my theory henceforth 

 upon the correctness of this assumption, I shall now endeavor briefly to 

 point out how the great currents of air, which are subject to the same 

 laws as the corresponding currents in the sea, move in the atmosphere. 



Just as I endeavored, in my former paper on marine currents, to show 

 that the first cause of those general currents must be sought in the fact 

 that the tropical heat, by warming the water of the sea, diminishes its 

 specific gravity to such an extent that its surface is forced to rise to a 

 greater height in the tropics than can be consistent with the equilibrium 

 of the water, thus I also believe that it is proper, generally, to assume 

 that the main currents in the atmosphere are caused by the fact that 

 the air, by reason of its diminished density under the equator, is forced 

 to rise to a considerably greater height than can be consistent with 

 equilibrium in respect to the masses of air which are outside of the 

 tropical regions. Because the height of the atmosphere within the trop- 

 ics is greater than it is elsewhere, the level surfaces in the upper por- 

 tions of the atmosphere of the northern hemisphere have an inclination 

 toward the north, and since the air must follow the inclination, this 

 upper portion of the atmosphere must move in a northerly direction in 

 the northern hemisphere. In this manner is produced an equatorial 

 current of air, which is called the anti trade-wind; but, during the move- 

 ment of this current toward the north, the force of the earth's rotation 

 makes itself felt and carries the movement more and more over in an 

 easterly direction. While the air under the equator thus rises and 

 rushes away toward the north [)ole and south pole in the upper portions 

 of the atmosphere, the colder and heavier masses of air, impelled by 

 the force of gravity, rush from the north and south along the surface of 

 the earth toward the equator, because the level surfaces in the loAver 

 portions of the atmosphere have an inclination toward the equator, since 

 the pressure of the air is greater outside of the tropics than under the 

 equator; but of course the rotation of the earth again exerts its influence, 

 and gradually changes the north wind, which seeks its way down toward 

 the equator, into a northeasterly wind, which we call the northeast 

 trade-wind, and concerning which I will here remark that it is probably 

 this colder current of air that, by its contact with the ascending calm 

 damp masses of air within the tropics, produces the violent hurricanes, 

 with their frightful electric discharges and immense falls of rain, which 

 occur iu West Indian waters. 



As the upper currents of air in the atmosphere get beyond the tropics, 



