48 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



(" cyclonic"). All areas of higher pressure than that of the 

 surrounding regions are invariably of the former class; all 

 areas of lower pressure than that of the surrounding regions 

 are invariably of the latter. 



III. Areas of depression tend to move in extra-tropical lat- 

 itudes with a more or less eastward progression. Areas of 

 high pressure, when of small extent, commonly follow the 

 progression of neighboring depressions; when of large dimen- 

 sions, progress with much less rapidity, and are frequently 

 erratic, and sometimes for a prolonged period stationary. 



IV. The direction of progression commonly varies in West- 

 ern Europe between north-northeast and south-southeast, and 

 is primarily dependent on the general antecedent distribu- 

 tion of surrounding temperatures, every depression area tend- 

 ing to advance at an inclination of about 45 toward the low- 

 er mean isothermals. This progression is, however, frequent- 

 ly interfered with, for, 



V. Mountainous districts, as well as certain coast lines, ex- 

 ercise (1) an attractive and (2) a detentive influence upon de- 

 pressions. 



VI. Extensive areas of very high pressure check, divert, 

 or accelerate the motion of depressions, every depression pro- 

 gressing with greatest facility in the direction in which it 

 lias the highest general pressures, on the right of its course 

 in the northern hemisphere, and the contrary in the south- 

 ern. 



VII. Depression areas are dependent, both for their origin- 

 al development and subsequent expansion, on precipitation, 

 which is also the medium through which the forces described 

 in propositions IV. and V. operate. Heavy and extensive 

 precipitation invariably precedes their first formation, and 

 accompanies their expansion, and its cessation immediately 

 precedes their collapse or dissipation. 



VIII. This influence of precipitation, as a disturbing or mo- 

 tive power in the lower regions of the atmosphere, common- 

 ly varies inversely as the general temperature of the atmos- 

 phere. 



IX. The upper currents of the atmosphere, while tending 

 in a general way to move with the highest pressures on the 

 right of their course, but depending in this respect on the 

 more extensive pressure systems, and being comparatively 



