188 ON SOARING FLIGHT. 



trating strata in a state of stable equilibrium is very marked, as seen 

 in the spreading and disappearance of smoke from locomotives and 

 furnaces, which, though possessing a very high temperature, rises more 

 and more slowly as it becomes diffused through the surrounding air. 

 The extent to which this diffusion is carried by nature we have no 

 means of ascertaining, but if it extends to the ultimate molecules of 

 the air the further rise of the heated air must be almost wholly checked. 

 The air, then, rising from innumerable points of the heated surface 

 of the field, is diffused through the overlying masses, whose tempera- 

 ture is thus slowly increased until a layer of constantly increasing 

 depth is brought to a condition of unstable equilibrium, while the air 

 above remains in the stable condition. The diffusion of a rising mass 

 of warm air will, I think, be very much less while passing through 

 strata whose equilibrium is unstable, for when the air is in the stable 

 condition it resists any force tending to displace it, and, if displaced 

 vertically in any manner, tends to return to its original position ; whereas 

 with an unstable equilibrium, if the air is displaced vertically, its 

 tendency is to continue moving in whatever direction it may be going, 

 whether up or down, and the displaced air, instead of retarding the 

 moving mass, tends to move on with it, so that the mass tends all the 

 while to increase in volume, and, meeting with little resistance, loses 

 little of its mass by diffusion. We are thus led to the singular conclu- 

 sion that the rise of a mass of heated air from the earth's surface is 

 most rapid while x)assing through the warmer stratum next the earth, 

 and that it is checked through diffusion on entering the colder air above. 

 As the accumulation of heat increases, the equilibrium will become 

 more and more unstable j bubbles and streams will rise with more vio- 

 lence, and will penetrate farther and farther into the cold air above 

 before being checked by diffusion, until at length the equilibrium will 

 be entirely destroyed, the cold air from the woodlands will press in on 

 all sides, winds will begin blowing in all directions, and the entire 

 mass of heated air will be drained away through some forced opening 

 in the cold strata above. Cold air will take its place, and the whole 

 process will be repeated. The above reasoning, which is based upon 

 the diffusion and viscosity of gases, is universallyapplicable under the 

 assumed conditions; and if we were to consider the case of a hill slope 

 instead of a plain the general course of the argument would be th© 

 same. The condition of unstabPe equilibrium, therefore, is the normal 

 condition, whereas if the tension thus brought about reach the requi- 

 site degree of intensity there will result the abnormally unstable con- 

 dition which it is believed by many meteorologists gives rise to cyclones, 

 thunderstorms, white squalls, and similar occurrences. Below are 

 given a number of quotations bearing upon the question under dis- 

 cussion, but in general having reference to the abnormal condition just 

 referred to. 



