50 



MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



3. Examines in which, icithin an area of low pressure, a new low center is developed by changes of 



pressure occurring icithin the low area. 



4. Examples in which, within an area of loio pressure showing several low centers, one or more of them 



disappears byichanges of pressure. 



It surely will not be claimed that in these cases the movement of the low centers can be 

 ascribed to a simple drifting of the general mass of the atmosphere in which the low areas were 

 formed. 



72. If we reject the drift theory it will doubtless be asked how can we explain the fact that in 

 the middle latitudes storms almost invariably advance toward the east, and the opposite move- 

 ment only occurs occasionally, and seldom continues longer than one or two days. This fact seems 

 to result from the prevalent movement of the wind toward the east, but the result is due, not to a 

 general drifting of the mass of the atmosphere within which^thelow area is formed, but to the fact 

 that the pressure on the west side of the low area is more steady and persistent than that on the 

 east side. The characteristic features of a groat storm movement iire a m.otion of the air from all 

 sides spirally inward, together with an upward movement, resulting in the condensation of vapor 

 at various places within the low area. Now if the air pressed in with equal force on all sides of 

 the low center, and if there was an equal precipitation of vapor on all sides, no reason is apparent 

 why the low center should advance at all. It sometimes happens that the pressure on the west 

 side is very small, while there is considerable pressure on the east side, and in such cases the low 

 center moves towards tiie west. Examples of this kind have been given in article 58. viz, Nos. 2, 

 3, 7, 15, 18, 27, 28, and 29 of Table XXII; and in article 62, viz, Nos. 25, 32, 33, 38, and 39 of Table 

 XXVIIl. But this movement towards the west cannot be long maintained. In the middle lati- 

 tudes the east winds are exceptional and result mainly from disturbances caused by storms. On 

 the contrary the west winds result from general causes, which are permanent in their character 

 and are independent of storms ; and if there were no storms the west winds would rarely be inter- 

 rui)ted. During the prevalence of an east wind the causes which i)roduco west winds are not 

 destroyed, their influence is only temporarily suspended, and they soon return with a force not 

 impaired but rather augmented by their temporary suspension. The pressure on the west side of 



Table XXIX. — Rate of progress q/ storm centers in the United States. 



