PHYSICAL PHENOMENA OF UPPER REGIONS OP ATMOSPHERE. 129 



the grating, appears on a white screen beside the ball. This is an 

 imitation of the spectrum analysis of the scintillation of the stars. 



We see by these few examples that the study of the optical phe- 

 nomena of the atmosphere aided by physical analysis and synthesis, 

 may and must teach us much concerning the calorific phenomena of 

 inaccessible regions. 



Dynamic phenomena of the atmosphere. — The phenomena we have 

 hitherto studied are due to states of almost complete equilibrium in 

 the atmospheric strata; we might call them static. But the calorific 

 action of the sun, combined with the cooling action of radiation into 

 space, may produce phenomena of movement presenting every degree 

 of intensity, from the weakest to the most violent. We will call these 

 dynamic phenomena. 



They are manifested under very diverse forms: 



1. Under the form of mechanical energy; which results in the forma- 

 tion of winds, whirlwinds, cyclones, waterspouts, etc. 



2. Under the form of calorific energy; which results in the formation 

 of clouds, rain, and hail, corresponding to the changes of state of water, 

 the ever variable element of the atmosphere. 



3. Under the form of electrical energy; lightning, thunder, etc. 



In fact, the transformation of solar energy into mechanical energy 

 is the fundamental phenomenon and the one that leads to all others. 

 For the sake of brevity this is the only transformation that we will 

 consider here. 



The most simple mechanical phenomenon that is produced in the 

 atmosphere is the wind. The wind has for its origin a difference of 

 pressure between two more or less distant points. We have known 

 since Pascal that the pressure of air is measured by the barometer. 

 We might, then, think that the direction of the wind could always be 

 determined by the indications of that instrument; that is to say, that 

 the wind ought to go from the point where the barometrical pressure 

 is greatest to the point where that pressure is least. 



But, strange to say, this is almost never the case ; the real direction 

 of the wind is always oblique to that theoretical direction. 



This fact has only been known for a few years. It has been put 

 beyond doubt by the general meteorological charts which, conceived 

 thirty years ago by Le Verrier, are to-day so widely circulated. 



The wind seems to move around the point in the chart where the 

 minimum is found, its direction, in the northern hemisphere, being the 

 reverse of that taken by the hands of a watch, or in the same direction 

 with the hands around the point of maximum pressure. In the south- 

 ern hemisphere these directions are reversed. 



In a word, the most ordinary movement of the atmosphere is a 

 gyratory one, that which is called cyclonic. 



This whirling movement of the air was noted long ago. We see it 

 occurring quite frequently around us; dust and dead leaves are raised 

 SM 96 9 



