408 ANNUAL OF SCIENTIFIC DISCOVERT. 



bodies .is largo as lakes or seas. Even large collections of the fluid evapo- 

 rated from the central nucleus, may be sometimes driven in this manner be- 

 yond the sphere of the comet's effective attraction ; and may separate for 

 ever from the primitive mass, taking away by its attraction much of the at- 

 tenuated matter composing the envelop. 



That the division of Biela's comet arose from a cause of this nature is 

 proved by a remarkable fact. The two parts into which this body divided 

 were almost equal in size and brilliancy, when nearest to the sun ; but at a 

 more considerable distance from him, one was about eight times as large as 

 the other, and about four times as bright. This shows that they differed 

 much in their capabilities of affording material for evaporation ; and it is 

 precisely what should occur if one were fluid and the other composed almost 

 exclusively of solid matter. On approaching the sun, the nebulous appen- 

 dage of the first should swell by the introduction of vapor, while the small 

 amount of vapor contained in the other would be only rendered invisible by 

 solar heat. Other comets manifest signs of similar commotions ; and we 

 cannot fail to recognize the close relation between the agencies operating on 

 our own globe and on the more humble members of the solar family. 



ON THE GRAND CURRENTS OF ATMOSPHERIC CIRCULATION. 



The following is an abstract of a new tlrcory of atmospheric circulation 

 brought before the British Association, at Dublin, by Mr. J. Thomson : 



It has been ascertained as a matter of observation, that in latitudes ex- 

 tending from about thirty degrees to the poles, the winds, while prevailing 

 from west to east, prevail also in directions from the equator towards the 

 poles. Now this motion towards the poles appears not to have been hitherto 

 satisfactorily explained. In fact it is the contrary motion to what is natur- 

 ally to be expected when the theory of Halley, which was given about the 

 year 1686, and which appears to affo'rd the true key to the explanation of 

 the trade winds, is followed up with respect to the circulation of the air in 

 other latitudes than those in which the trade winds occur. According to this 

 theory so applied, it would naturally be expected that the air, having risen 

 to the upper regions of the atmosphere in a hot zone at the equator, should 

 float towards the north and south polar regions in two grand upper currents, 

 retaining, as they pass to higher latitudes, some remains, not abstracted by 

 friction and admixture with the currents below, of the rapid equatorial 

 motion of about 1,000 miles per hour from west to east, which they had in 

 moving with the earth's surface at the equator. Also, it would be expected 

 that the air in the polar regions should have a prevailing tendency to sink 

 towards the surface of the earth, in consequence of its increased density 

 caused by cold ; and that it should tend to flow from the polar regions along 

 the surface of the earth, towards the equator, with a prevailing motion from 

 west to east in advance of the earth, until, by friction and impulses on the earth's 

 surface, the motion in advance of the earth is retarded and counteracted. The 

 theory, however, thus deduced, is found in one essential point to be controverted 

 by observations. This point is what was stated in the outset of the present arti- 



