MOVEMENTS OF ATMOSPHERE GULDBERG AND MOHN 139 



an active force which can produce the movements of the particles 

 of air. The action of the heat of the sun is presented under two 

 different forms; on the one hand it produces the changes of the 

 temperature of the atmosphere and on the other hand by the evap- 

 oration of wat^r it produces changes in the mass of the atmosphere. 

 The direct action of these phenomena is to produce changes in the 

 pressure of the air accompanied by movements of the particles of air 

 which give rise to the currents of air. 



The currents of air, which can have any direction whatever, tend 

 always to destroy the perturbations and to produce a new state of 

 equilibrium. We can imagine permanent currents in the atmos- 

 phere; let us suppose that a continuous heating takes place at one 

 point and that a cooling takes place at another, we see that there 

 will arise two currents, one carrying warm air and the other cold 

 air. 



The heat set free in the atmosphere in any way whatever, produces 

 currents of air. We notice the currents of air during a forest fire, 

 and during the eruption of a volcano. In the last case the vapors 

 and the shower of ashes set free the heat. We take this occasion to 

 remark that during the eruptions of volcanoes and during earth- 

 quakes it is probable that masses in the interior of the earth change 

 their positions. If the masses are great enough to influence local 

 gravitation we can explain the formation of the currents of air, by 

 supposing that the displacement of the masses in the interior of the 

 earth produces a sudden change in the force of gravity. This 

 change will be accompanied by a rapid change in the pressure of the 

 air which will produce currents of air. 



Chapter II 



PERMANENT AND HORIZONTAL CURRENTS OP AIR 



§7. Isobars and gradients 



During the movement of the air certain new forces come in play 

 and the level surfaces of § 6 are no longer surfaces of equal pres- 

 sure. Let us consider a surface of equal pressure, or an isobaric 

 surface, during such movement; this surface cuts the level surfaces 

 into lines that are called isobaric. We shall occupy ourselves here 

 principally with the isobars at the surface of the earth. 



In considering the variation per unit of length of the pressure at 

 any point we perceive that the variation along the isobar is noth- 

 ing and that the variation has its maximum value along the normal 

 to the isobar. 



