136 METEOROLGT. 



rial day, forming, in consequence of the centrifugal force, the exterior 

 of a spheroid considerably pioluberant at the equator. Tliere would 

 be a dead calm ; for there would be no cause to excite currents or 

 "winds. To assert, (as some philosophers havedone,) that the rotation of 

 the earth on its axis alone would produce currents, is equivalent to as- 

 serting that motion could be produced without a cause. Were even the 

 velocity of the earth suddenly altered, the atmospliere would soon par- 

 take of that alteration, and a mean motion would arise, which would be 

 the same as if the impulse had been communicated to the whole mass. 



But that rest of the atmosphere is constantly disturbed by the varia- 

 ble quantities of solar heat communicated, first to the surface of the 

 earth, and then to the air in contact. The alteration in the density of 

 the heated air, produces vertical currents, by which the mass becomes 

 heated upwards to a certain distance in each latitude and climate, and 

 horizontal cnrrents, by which, by a constant interchange, the extremes 

 of heat and cold are obviated. 



There are changes of temperature, which, being local or confined to 

 comparatively small territory, give rise to local winds, and which are 

 violent in proportion to the rapidity of the alteration in temperature and 

 density of the air. Thus, the rapid heating by day and cooling by 

 night of the surfaces of islands, produce, in the former case, the sea- 

 breezes, and in the latter the land-breezes. 



There are other changes of temperature, which, being general, or 

 extending over a large territory and more permanent than the former, 

 give rise to general currents of considerable duration, or cause a deter- 

 mination of a large mass of the atmosphere, for some time, to be in one 

 direction. Thus, the greater heat of the tropics causes the air of the 

 higher latitudes to press toward;? the Equator, and, aided by the earth's 

 rotation, to produce the trade-winds. Also, during our summer, there is 

 an fflux of air towards our continental land, from the two great eastern 

 and western oceans, and during winter an efflux from the land towards 

 the oceans. This is similar in kind to the sea and land-breezes in insu- 

 lar situations, only that t!ie changes are more gradual and the currents 

 more gentle, and that they are not daily, but annual. 



Keeping these laws of disturbance in view, we may the more readily 

 determine when the air is in its normal condition of rest, or at least of 

 approximate rest. 



Firsf, in insular situations, a calm or rest in the atmosphere precedes 

 the sea-breeze in the morning, and the land-breeze in the evening. In 

 like manner ought we expect it to be in tlie great annual continental land 

 and sea-breezes ; a calm or comparative rest ought to precede the win- 



