THE BIRTH, LIFE, AND DEATH OF A STORM. 687 



sure over a given area, and the only reason why storms are generally 

 associated with cyclones is that these systems afford us the most seri- 

 ous instances of disturbances of atmospheric equilibrium, and conse- 

 quently of differences of pressure, which are met with on the globe. 



At any place where an area of relatively high pressure comes into 

 close proximity to an area of relatively low pressure, a gale will result, 

 and so a storm may be due just as much to the rise of the barometer 

 in one region as to its fall in an adjacent district. For the same physi- 

 cal reason, however, that the eddies in a river extend downward, and 

 the water does not pile itself up in a peak, the normal disturbance of 

 atmospherical equilibrium is the appearance of one of these vortices 

 with pressure decreasing rapidly toward the center. Wherever there 

 is a rapid decrease there is a steep gradient, and consequently a strong 



wind. 



Defining the term cyclone, in its very widest acceptation, as indi- 

 cating a region of diminished pressure, round and in upon which the 

 air is moving along paths which are more symmetrical all round the 

 center the more perfect is the circular form of the system, we must at 

 once see that not every cyclone is accompanied by a storm. The fact is, 

 that the direction and force of the wind are regulated by the difference 

 of barometrical pressure over a given distance, and not in any way by 

 the actual height of the barometer at the station at which the storm is 

 felt, or by the distance of that station from the point where the baro- 

 metrical reading for the time being is the lowest. 



This explanation of wind-motion is almost the only new principle 

 which has been recognized in our science during the present genera- 

 tion, and its practical importance is daily forcing itself more and more 

 into public notice with the development of weather telegraphy. It is 

 usually known under the name of Buys Ballot's Law, and is stated as 

 follows : " Stand with your back to the wind, and the barometer will 

 be lower on your left hand than on your right." The truth of this 

 law is evident to any one who looks at a weather chart ; but the Dutch 

 Professor, after whom it is named, though he justly claims the credit 

 of having persistently advocated the acceptance of this relation of the 

 wind to the distribution of pressure, was not by any means the first to 

 discover it. 



The final result of all the inquiries into the question is, that on the 

 mean of all winds the angle between their direction and the tangent 

 to the isobar at the place is about 20. 



These principles of wind-motion have a most important bearing on 

 the theory of the motion of the air in hurricanes and typhoons. The 

 old popular idea of these phenomena is, that the air blew round and 

 round the central calm in circles, so that any sailor caught in one of 

 these storms could at once know that when he was hove-to, if he looked 

 in the wind's eye, the center bore eight points to the right in the north- 

 ern hemisphere, and to the left in the southern ; or, what is the same 



