190 ON SOARING FLIGHT. 



surface or at some locality above it. The mass of air, being warmer 

 than tlie surrounding air at the same level, is in unstable equilibrium, 

 and when some slight disturbance frees it from its abnormal position 

 it is forced upward by the pressure of the air below and around it." 

 " Heat thunderstorms are the result of the local heating of the lower 

 air, which makes its condition unstable." 



The same author, commenting on the great Paris storm of September 

 10,1896, says: "We know that when air is compressed it becomes 

 warmer, and when it is expanded it becomes cooler, even though no 

 heat be added or subtracted from the air mass. And the change, called 

 adiabatic change, proceeds according to a regular law. The air pres- 

 sure, and consequently the air density, decreases with the increase of 

 altitude above the earth's surface, and so when air moves upward it 

 expands and becomes cooler, at the rate of about 1 degree Fahrenheit 

 for each 183 feet of ascent; and likewise it becomes warmer 1 degree 

 for each 183 feet of descent in cases where it moves downward. 



"So, then, if the temperature of a mass of air decreases 1° Fahren- 

 heit for each 183 feet of increase in altitude, then the air is said to be 

 in indiiferent equilibrium, and any air carried upward or downward in 

 it will remain in its new position, because its adiabatic change of tem- 

 perature has been just such as to allow the air so moved to accommo- 

 date itself to the temperature of the surrounding air in its new position. 



"If a mass of air decreases at a rate of less than 1° Fahrenheit for 

 each 183 feet of increase in altitude, then the air is in stable equilibrium, 

 and if any air is forced upward in it, it would gradually become denser 

 than the air at its level and would sink back again to its starting place 

 after the force which had caused it to move upward had ceased to act. 



"If the mass of air decreases in temperature at a rate greater than 

 1° Fahrenheit for 183 feet of increase in altitude, then it is in unstable 

 equilibrium, and if any of the air is started upward or downward it 

 will continue so to move, as it will become lighter than the surround- 

 ing air with the upward, and heavier with the downward motion. It 

 is on this condition of unstable equilibrium that most squalls depend 

 for their origin and in great part for their maintenance." 



Dr. Buchan, in the " Encycloptedia Britannica," says: "Whirlwinds 

 occur where for the time the air is unusually calm and moist, and 

 where, consequently, temperature and humidity diminish with height 

 at an abnormally rapid rate. Whirlwinds and tornadoes have their 

 origin in vertical disturbance of atmospheric equilibrium." 



The following extracts are from the writings of Dr. Ferrel, whose 

 theories of cyclones and tornadoes have perhaps met with more general 

 acceptaoce than those of any other writer. Even Dr. Hann, who advo- 

 cates a different theory of cyclones, seems to accept his theory of the 

 formation of tornadoes, whirlwinds, and similar smaller disturbances 

 in the air: 



"The principal condition of a tornado is the unstable condition of the 



