42 REPORT — 1859. 



affected by an increase of latitude, and must therefore operate with less energy in 

 causing a general circulation of the atmosphere. 



My researches show that the chief obstacle to the extension of trade-winds to the 

 temperate zones, proceeds from the diurnal motion of the earth. On the centrifugal 

 force arising from this rotation depends, to some extent, the direction of terrestrial 

 gravity at places between the poles and equator; the equilibrium of our atmosphere 

 is accordingly dependent on it; and if this vast collection of air ceased to partake of 

 the earth's movement, the greater part of it would be compelled to remove from the 

 tropical to the circumpolar regions. In like manner, when the air is moving towards 

 the west, it experiences a reduction of centrifugal force, accompanied by a slight 

 change in the direction in which it is attracted by our planet, and a proportionate ten- 

 dency to flow towards the pole ; while in an eastward movement the effect would 

 be reversed, and there would be a steady deflection towards the equator. 



Whenever an extensive portion of our atmosphere undergoes a considerable change 

 of latitude from local variations of temperature, it cannot, at once, acquire the velo- 

 city and the centrifugal force necessary for an equilibrium in its new location, and a 

 retrograde movement is a necessary consequence. Accordingly the earth's rotation, 

 instead of disturbing the repose of our aerial ocean, only imposes restraints on the dis- 

 turbances arising from the action of solar heat. Its resistance to atmospheric move- 

 ments (supposing friction removed, and the motive power to act in the direction of 

 the meridian) is nearly proportional to the square of the sine of latitude multiplied by 

 the distance the air has been withdrawn from the parallel of the place which has the 

 same velocity with respect to the earth's axis. This rate of variation appears to be 

 approximately correct, whether the air be supposed to preserve its eastward velocity 

 unchanged in its passage towards the equator or the poles, or whether cognizance is 

 taken of the change of velocity, with which the translation must be attended, that 

 our globe may sustain no loss of momentum by aerial commotions. But in the latter 

 case a higher coefficient of resistance will be obtained, though it must be diminished 

 in consequence of the effects of friction. It thus appears that the centrifugal force 

 attending the rotation of our planet, impedes only in a slight degree the extensive 

 movements of the winds in tropical regions ; but it becomes a serious impediment to 

 their prevalence on the same scale, in the temperate and the frigid zones. 



As the part of the atmosphere which feels most intensely the expanding influence 

 of heat is compelled to ascend, in the vicinity of the equator it flows towards the poles, 

 forming two vast aerial rivers, whose breadth is nearly 25,000 miles at their origin, 

 but is reduced to about 22,000 miles on reaching the parallels of 28 degrees. Such 

 a reduction of breadth would evidently be accompanied with an increase in the depth 

 of the stratum of air, were it not for the decline of temperature ; but it cannot fail to 

 augment atmospheric pressure, especially at the place where the progressive move- 

 ment from the equator is arrested by the resistance from centrifugal force. This 

 occurs between the 25th and 30th parallels of latitude ; and here the great pressure, 

 of which the barometer gives manifest indications, causes the air to descend, to roll 

 back towards the equator, and to participate once more in the circulation of the trade- 

 winds. It appears, moreover, that the mere form of the earth must be an impe- 

 diment to the extension of trade-winds to any considerable part of the temperate 

 zone, where the degrees of longitude diminish so rapidly in length; for the belt of 

 air which encircles the equator could not make a general movement as far as the 60th 

 parallel of north latitude, without swelling to a height wholly incompatible with the 

 conditions of equilibrium. 



The aqueous vapour conveyed by the trade-winds is condensed into rain during 

 the ascent of the air at or near the equator ; and the evolution of heat attending this 

 condensation must, according to Professor Espy, be regarded as the chief source of 

 power which maintains the great circulation of the tropical atmosphere. He has 

 long ascribed storms to the local condensation of vapour, and he adduces evidence to 

 show that the winds blow to the point at which the most heavy rain is descending ; 

 but Dr. Hare attributes this centripetal movement to the constant discharge of elec- 

 tricity, which the moisture of the air enables to escape from great elevations. Now, 

 whatever part heat and electricity may act in these phenomena, the results must be 

 modified in the same manner by the diurnal motion of our globe. As the impedi- 

 ment which centrifugal force gives to atmospheric movements augments with every 



