B. TERRESTRIAL PHYSICS AND METEOROLOGY. 49 



unaffected by very limited baric areas, yet deviate consider- 

 ably from Ballot's Law ; for, 



X. Upper currents manifest, in a large percentage of ex- 

 amples, a distinct centrifugal tendency over the areas of low 

 pressure, and a centripetal over those of high. 



XL The axis of a progressive depression commonly inclines 

 backward. 



Several of these propositions are, however, according to Mr. 

 Ley, ultimately dependent upon the following primary law, 

 which, although obvious, requires to be clearly apprehended 

 at the outset by the student of meteorology. "Every ex- 

 tensive centripetal motion in the atmosphere tends to become, 

 through the influence of the earth's rotation, a helix, the cur- 

 rents of which are retrograde in the northern hemisphere and 

 direct in the southern. Every extensive centrifugal motion 

 tends to become helix, the currents of which are direct in the 

 northern hemisphere and retrograde in the southern." 



Also, first, that extensive precipitation occurring in a re- 

 gion of atmosphere previously approaching a condition of 

 tranquillity is the primary factor of every system of baric de- 

 pression, with its resulting atmospheric circulation, retro- 

 grade in the northern and direct in the southern hemispheres ; 

 second, that such an atmospheric circulation being establish- 

 ed, the changes in their capacity for aqueous vapor which its 

 currents undergo in consequence of the unequal distribution 

 of solar heat tend to propagate the depression in an eastward 

 direction. 



To the subject of" upper currents" a special chapter is de- 

 voted, and the difficulties of making observations upon them 

 is referred to. The special object of this inquiry is to ascer- 

 tain whether there is any general relation between the mo- 

 tion of this upper stratum and the conditions and disturb- 

 ances of atmospheric pressure at the surface of the earth ; and 

 if so, what that relation is. As a partial answer to these in- 

 quiries, resulting from the discussion of numerous observa- 

 tions, the author remarks that the relation between the num- 

 ber of instances in which the upper currents incline from low 

 to high pressures, and that in which they incline from high 

 to low, is as 393 to 92 (or about four to one). 



We thus arrive at the important general law connecting 

 the direction of the higher currents with the distribution of 



C 



