470 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



These are the formulae which ought to be applicable in studying the 

 rotatory movements of the air in cyclones and storms. We have already 

 stated, at the beginning of this memoir, that the laws which we have 

 previously enunciated apply to the movements of the air in tempests, 

 and that they have been already recognized and formulated by numerous 

 observers. We must, then, conclude that, even in meteorological phe- 

 nomena, where the apparent movement of the air is most irregular and 

 violent, the composite centrifugal force retains a preponderating influ- 

 ence, such that the variations of barometric i^ressure are controlled, 

 principally by this force, according to the laws that we have developed. 



But if our formute give the laws of tempests as a particular case, it is 

 important to remark that they are more general still, and that they are 

 applicable to all the movements of the air in our hemisphere. It is not 

 necessary that there should be a storm (that is to say, a rotatory move- 

 ment of the atmosphere accompanied by rain) in order that the direction 

 of the winds should obey the prec«ling rules. Every terrestrial region 

 where a barometric depression is ])roduced will be surrounded by winds 

 circulating in a direction opposite to that of the hands of a watch ; and 

 conversely, if the barometric pressure is high over any region, the winds 

 prevailing upon the borders of this region will circulate in the same 

 direction as the hands of a watch. It is only necessary to glance upon 

 the synoptic charts in order to ascertain how fully these rules as to the 

 direction of the winds are confirmed by the observations that are therein 

 presented. 



(G) Explanation of the Variation of Barometric Pressure with the Latitude. 



Our theory permits us to give an explanation of some of the facts, 

 generally accepted, relative to the variations of barometric pressure 

 upon the surface of the earth. 



The barometric pressure reduced to the level of the sea is not con- 

 stant, but varies with the latitude. Its mean value at the equator is 

 about 758 millimeters. It increases from the equator to a j^arallel be- 

 tween 3 GO and 40°, and there attains a maximum value of 7G2 to 7G4 

 millimeters. If we proceed to higher latitudes, the pressure diminishes, 

 and in the polar regions it is probably less than 756 millimeters.* This 

 variation of pressure with latitude is also easily confirmed by the syn- 

 optic charts. On each chart we see designated a region of high pressure 

 whose center is generally in the middle of the Atlantic near the parallel 

 of 30°. The pressure at the center often attains 775 millimeters, and 

 sometimes 780. The winds are feeble near the center, but on the borders 

 of this region they become stronger. On the east and south borders, 

 they blow from points of the compass comprised between north and east, 

 and constitute the trade-winds. On the northern border, the winds blow 

 very generally from the west, and are connected with the anti-trade 

 currents. 



* Kiimtz's Meteorologie, Martin's editiou, p. 258. 



