ON SOARING FLIGHT. 189 



THE ORIGIN OF CYCLONES, THUNDERSTORMS, AND SIMILAR DIS- 

 TURBANCES, AND THEIR RELATION TO SOARING FLIGHT. 



The following account of experiments to determine the permeability 

 of the air by vapor of water is taken from Espy's Eeport on Meteor- 

 ology, 1849: 



^' I next took a glass tube about 2 feet long, with its internal diame- 

 ter one-third of an inch, bent at one end into the form of a shepherd's 

 crook, and hermetically sealed at the short end. The open end was 

 plugged with chloride of calcium, the outer end of the calcium being 

 excluded from the atmosphere. A film of air touching the chloride of 

 calcium would be made perfectly free from vapor at one end of the tube; 

 at the other a film of air touching the water would be as near satura- 

 tion as evaporation could make it; and at a temperature of 80 degrees 

 the pressure of the vapor at one end of this tube was near half a pound 

 to the square inch, and at the other nothing at all. With the tube set 

 vertically, so that the vapor of water would reach the calcium chloride 

 plug by ascending, the evajjoration at the end of three months, with a 

 daily temperature of about 70 degrees, amounted to but one-eighth of 

 an inch." From this and similar experiments Mr. Espy inferred, "con- 

 trary to the general belief of scientific men, that vapor permeates air 

 from a high to a low dew point with extreme slowness, if indeed, it per- 

 meates it at all ; and in meteorology it will hereafter be known that 

 vapor rises into the regions where clouds are formed only by being car- 

 ried up by currents of air containing it." Incidentally, the experiment 

 seems to prove that convection also took place very slowly; for the 

 result would have been the same whether the vapor reached the calcium 

 chloride by permeation or convection. 



The following abstract is from an article on meterology in the New 

 American Cyclopaedia: "The explanation, or at least the approximate 

 cause of the fall of rain and of fitful winds, is found in the unstable 

 condition of the atmosphere produced by the introduction into the 

 lower strata of the vapor of water. This, with the accompanying heat, 

 tends to expand the air, and consequently to render it lighter; and 

 when the amount of vapor becomes safificieutly great the order of den- 

 sity is reversed and a state of tottering equilibrium is ]Droduced, the 

 lower stratum tends on the least disturbances to break through into 

 the colder." 



The following statement is taken from Modern Meteorology, by Dr. 

 Frank Waldo: "Where the addition of heat takes place too rapidly 

 and the gradient exceeds the theoretical value, then the condition of 

 unstable equilibrium ensues for a short time; such being the condition 

 which Eeye and others have assigned to tornadoes and thunderstorms." 



In Elementary Meteorology, he says: "The principal condition for 

 the formation of a tornado is the local unstable condition of the air, 

 due to the abnormal heating of a mass of air either at the earth's 



