METEOROLOGY. 539 



5. This atmospheric current finding no outflow in the closed valleys, 

 must fill up the latter to the level of the surrounding ridges, flowing 

 from one valley to the next until it reaches the limit of the mountainous 

 area. 



6. The bottom of such an outward flow of air is therefore at the level 

 of the ridges of the mountains. Below this there prevails in the valleys 

 calm and fog and low temperatures. 



7. Above this limit prevail cloudless skies, and the air sinks from 

 regions of slight pressure down to levels of higher, and notwithstand- 

 ing the steady rise in its temperature it falls lower, step by step, to 

 the foot of the mountains and thence outward, distributing a compara- 

 tive warmth throughout the low lands. 



8 and 9. When the descending and outflowing masses of air are 

 hindered by outlying mountain ridges they pile up to a level sufficient to 

 give them impulse to further outflow. This level forms then a dividing- 

 surface like that over the valleys within the mountains ; whence fol- 

 low also similar temperature anomalies, but inequalities in the surface 

 of equal pressure are wanting, as also the gradients that give rise to 

 increase in movements. 



10. The progressive rise of the equal pressure surfaces over the 

 mountains up to the neutral surface forms a hindrance in the upper 

 depression, in consequence of which the center of maximum pressure 

 must move from the mountain system towards the latter. (Z. 0. Q. 

 ,1/., xvn, p. 214.) 



Dr. Hann, in some remarks on the study of movements of barometric 

 maxima and minima, proposes to call these the chief centers of ac- 

 tion of the atmosphere, and the regions of the earth covered by such cen- 

 ters the chief centers of action of the earth's surface. The importance of 

 considering these centers seem to have been fully appreciated by Taste 

 in a memoir read before the Paris Academy of Sciences in September, 

 1871 ; but the importance of the movements of these centers can only be 

 appreciated by the study of the daily weather charts, whence it results 

 that besides the movements of centers of depression there are also 

 changes in the location of the principal belts of maxima, such as those 

 of the tropics, which, of course, affect the weather over large portions 

 of the continent. Hann regrets that the rapid progress recently made 

 in the study of the daily weather charts for small portions of the earth's 

 surface has lessened the esteem in which students at present hold the 

 study of monthly and annual means over large portions of the earth's 

 surface. He maintains that most important insight into the causes of 

 long-enduring abnormal departures of temperature, pressure, and wind 

 are to be obtained by mean charts for each month over large portions 

 of the earth, and that to this there should be an international co-opera- 

 tion for the increase of stations in lower latitudes ; that, in fact, the key 

 to the weather of the temperate zones lies in the tropics and subtropics, 



