INTRODUCTION. XX111 



considered alone, would cause the barometer at the very poles to 

 stand above the mean ; but the centrifugal force of the air in the 

 frigid zone, arising from the diurnal revolution of the earth on her 

 axis, produces a countervailing force of considerable power, and 

 flattens the atmosphere at the poles more than the earth is flattened, 

 because it is of greater diameter, and perhaps this may cause the 

 barometer to stand below the mean very near the poles. Let this 

 matter be further examined. As the air in the torrid zone cer- 

 tainly does rise and run out towards the poles above, it must rise 

 there in columns and not all over the' zone at once, otherwise this 

 whole zone would be covered with eternal cloud, formed by the 

 condensation of the vapor from the cold of diminished pressure as 

 the ais ascended. 



If violent storm clouds, which necessarily rise to a great height 

 in the upper current, are driven forward in the direction of the 

 upper current, it is probable that the barometer will rise higher in 

 that part of the annulus which is in front of the storm, than in the 

 rear, and if so, a sudden rise of the barometer, in particular local- 

 ities, may become, when properly understood, one of the first 

 symptoms of an approaching storm. (116 Rationale, 170.) In 

 consequence of the high barometer in front of the storm in a semi- 

 annulus, the air will be forced downwards there, and cause, in 

 some cases a more violent action of the air or wind backwards, 

 meeting the approaching storm, than will be experienced in the 

 rear of the storm. As the barometer will probably be highest in 

 the centre of the semi-annulus, north east of the storm, in middle 

 latitudes, the tendency of the wind to blow outwards on all sides 

 from the region where the barometer stands highest, may cause 

 the wind in the beginning of the storm to blow so as to appear to 

 whirl from left to right, on the east side of the storm, and from 

 right to left on the west side. (p. 240.) 



As the air comes downwards in the semi-annulus in front of the 

 storm, it will come urlder greater pressure, and therefore any 

 clouds which it may contain will probably be dissolved by the 

 heat of greater pressure ;* consequently, on the passage of the an- 

 nulus it will probably be fair weather, (p. 241.) Also, as the air 

 above always contains more caloric to the pound than the air be- 

 low, more especially that which has just spread out above from a 

 storm cloud, containing the caloric recently evolved from the con- 

 densing vapor, there will be an increase of temperature on the 

 passage of the annulus, partly from the increased pressure, partly 

 from the increased radiation of the hotter air above, (p. 288) but 

 chiefly by the descent of the air itself. In very hot climates, this 



1 An air tight piston, moved in a glass cylinder, will cause a cloud to appear, 

 by rarefying the air; and to disappear, by condensing it. 



