186 NINTH ANNUAL REPORT OP 



on which all atmospheric disturbances are to be studied. First, that 

 the atmosphere can only remain in a state of rest or equilibrium when 

 the barometer stands at the same height at the level of the sea in all 

 j)arts of the globe, because the aerial envelope lias a tendency like water 

 to seek its equilibrium. Second, that the atmosphere must every where 

 have the same temperature at the level of the sea, and that its various 

 strata as we ascend must have a temperature corresponding to their 

 position — that is, that every atom, from the top to the bottom of the 

 atmosphere, must possess the same absolute amount of heat. 



A number of natural agencies are at work to disturb the equili- 

 brium ot the atmosphere, and to give rise to aerial currents; among 

 them the most important is the difference of temperature in different 

 parts of the earth. The air within the tropics, constantly heated by 

 the rays of an almost perpendicular sun, is rendered ligliter, and is 

 pushed upward by the heavier air north and south of this region. A 

 current in this direction from each pole is thus produced at the surface 

 ol the earth, while an opposite current towards each pole is gcner§ted 

 by tlje rarefied air which rises above the heated belt, and flows back- 

 ward like water seeking its equilibrium. These currents, on account 

 of the rotation of the earth, are not along the meridian, but those at 

 the surface take a westerW direction, while those above flow in an 

 easterl}^ course. This is the origin of the trade winds at the surface 

 of the ea'th, and of the great westerly current which is almost con- 

 stantly moving in the upper strata over the middle and northern por- 

 tions of the United States. That such a current does prevail over the 

 regions mentioned is clearly proved by Professor Coflin^s admirable 

 report on the winds of the northern hemisphere, published in the Smith- 

 sonian Contributions to Knowledge. The phenomenon of the con- 

 stancy of this upper current early attracted m}'- notice in my tour 

 through this country, and I learn from the records that it is the same 

 all the 3^ear. The fact of the existence of this current is referred to 

 by President D wight, in his History of New England, to explain some 

 atmospheric phenomena; and, indeed, it is one of the keys to a know- 

 ledge of the peculiarities of the meteorology of this country. 



To understand some of the peculiar actions which occur in the lower 

 strata of the atmosphere, it is necessary for us to consider a little more 

 attentivel}^ the effect which sometimes takes place when a large area is 

 slowly heated and the air above it gradually expanded. In this 

 case the heated air, increasing its volume and resting on the surface of 

 the earth, pushes up the air above it, and thus retains it in a state of 

 unstable equilibrium. This condition was observed by the French 

 savans as existing over the heated sand ot"the desert, and giving rise to 

 the mirage. It was also observetJ by Colonel Sykes, on the plains of 

 Hindostan, and is quite common in all latitudes. 



This is a very unstable condition of the atmosphere, and is constantly 

 liable to be overturned ; yet its philosophy is not difficult to comprehend. 

 When the atoms of air in the lower stratum are gradually and ecjually 

 heated, all have a tendency to rise, and the cold atoms above have a 

 tendency to descend. But as there is not room for all to descend and 

 all to ascend at the same time, there is little downward or upward 

 motion. 



