308 ROCHESTER ACADEMY OF SCIEXCE. [Mav 28, 



whirls which we see in the streets on a breezy day are similar phenom- 

 ena in miniature. 



To the cyclones we are indebted for the extreme changeable- 

 ness of our weather, if not to the existence of any " weather" what- 

 ever. They are very powerful, wonderful and interesting phenomena. 

 A general understanding of their character and movements gives a 

 new source of interest in nature, and the weather becomes a subject 

 of fascinating study, as a matter of obedience to physical laws and not, 

 as formerly regarded, a mere chance or haphazard thing. "The 

 wind bloweth where it listeth * * * "is not true today, for we 

 know the laws which control the atmospheric movements, and within 

 rather narrow limits we can predict the changes. The telegraph 

 enables us to quickly collect the weather data from any number of 

 stations over the continent, and from the daily or semi-daily compari- 

 son we can prognosticate for one or two days ahead. 



Cyclones are ascending vortices of warm and lighter air. They 

 originate in regions where the lower air becomes heated by contact 

 with the heated earth surface. The effort of the lighter air to break 

 through the overlying cooler layer produces a vortex which is partly 

 illustrated in a contrary direction by the whirl of water running down 

 through a hole. In the northern hemisphere the cyclones have a con- 

 tra-clockwise revolution, (Figure i) while their progression or direc- 

 tion of passage (translation) across the continent is generally eastward, 

 in the great earth-encircling current of middle latitudes (Figure 2). 

 The barometric pressure in the cyclone decreases toward the center, 

 which is called the "low." On the weather maps the different pres- 

 sures, or barometric gradient, are shown by the lines of equal pressure, 

 called " Isobars." These represent differences of one-tenth of an inch 

 of the mercury column, and properly form concentric circles (Fig. 2). 

 The cyclones may have a breadth of even 1000 or 1500 miles, cover- 

 ing half the width of the continent. In the velocities of rotation and 

 progression, and in all other respects, they greatly vary. 



Surrounding the cyclones are areas of descending cold and heavy 

 air. The mass of heavy, cold air following after the cyclone with a 

 generally clock-wise movement is called the " anticyclone" and is the 

 "cold wave" with high ])ressure. On the maps the centers are 

 marked "highs." When the cyclonic movements are active, in the 

 winter months especially, these "lows" and " highs" chase each other 

 in more or less regular succession across the continent. 



A steadily falling barometer indicates the approach of a "low," 



