14 Dr. Hall on one of the Causes 



Art. III. — On One of the Causes of the Movements of the 

 Barometer, and of the South and West Winds. By Mar- 

 shall Hall, M.D., F.R.S.E., 8?c. 8?c. 



[Communicated by the Author.] 



Our inquiries into the nature and causes of the changes in the 

 atmosphere will be greatly facilitated, by having first apprehended 

 its more usual and quiescent state. The following observations 

 will therefore be properly introduced by a very short description 

 of what may be termed the natural state of the atmosphere. 



If the surface of the globe were even and uniform, unchequered 

 by mountains and valleys, and unintersected by rivers and seas f 

 the waters of which continually evaporate and recondense, the. 

 atmosphere would remain in a comparatively tranquil state ; and 

 those movements which did take place in it, being only excited 

 by the influences of the sun and moon, would be regular and 

 periodical, and would consist chiefly in a constant wind from the 

 north-east in the northern hemisphere, and in a diurnal oscillation 

 or atmospheric tide. This movement of the atmosphere from the 

 north-east is produced in the following manner. The sun, acting 

 powerfully on the surface of earth within the torrid zone, heats 

 and rarefies the superincumbent air, and causes it to rise into the 

 upper region of the atmosphere ; the place of this air is supplied 

 from the poles, and thus, in the northern and southern latitudes, 

 a north and south wind is produced respectively : as these portions 

 of air have, however, little or no other motion except that in the 

 direction towards the equator, and as the surface of the earth as 

 we approach the equator has more and more motion from west to 

 east, from its rotation on its axis, it follows that the air so 

 brought from the poles, must, as it approaches the equator, have 

 more and more of an apparent motion westward. These two 

 motions being combined, the air will pursue the course of the 

 diagonal, and to the inhabitants of the northern hemisphere of 

 the globe, will constitute a perpetual north-east wind. 



Such is the principal effect of the sun's influence on our atmo- 

 sphere. A further influence of this celestial body, and of the 



