PATHS OF MOVING PARTICLES SPRUNG 77 



Since the horizontal forces conditioned on the axial rotation of the 

 earth are of the same order as the vertical ones just considered 

 (equation 22) and become equal to* them at the latitude 45 , there- 

 fore the question arises, how comes it that the first are of such great 

 importance in nareteorology ; the reason lies simply in this that much 

 greater dimensions come into play in horizontal directions. It 

 frequently happens indeed that the whole region between the Alps 

 and southern Scandinavia is occupied by one and the same current 

 of air, in which the difference of pressure on the two sides of the cur- 

 rent (measured in a horizontal direction perpendicular to the iso- 

 bars) amounts to 30 or 4o mm . From this there results a "Gra- 

 dient" of 2.5 to 3-3 mm (for the unit length of one equatorial degree 

 or in km.), whereas for a distance of ij km. (corresponding to the 

 vertical distance above considered between Breslau and Schnee- 

 kopfe) there. results a proportional difference of pressure of o.o3Q mm , 

 a quantity that is no longer measurable with our barometers. 

 Therefore if at about latitude 50 a parallelopiped of air extending 

 from west to east of 1.5 km. height and breadth and previously at 

 rest were set into a condition of stormy motion then the simul- 

 taneous difference of pressure for the surfaces lying opposite each other 

 in a horizontal as well as a vertical direction, must change by about 

 o.o4 mm . Inversely the production and maintenance of such an 

 insignificant difference of pressure would suffice to gradually bring 

 about these same stormy motions; but the fundamental fact is that 

 the horizontal difference of pressure suffices for this purpose and 

 we should conceive the processes as going on toward completion in 

 the following order: 



At the start the motion of the air takes place in the direction of 

 the gradient, but this is departed from more and more with increas- 

 ing velocity and diminishing acceleration of the wind, until the 

 direction of its motion when it lias become uniform finally stands 

 perpendicular to the direction of the gradient or at least approaches 

 this perpendicular direction to a certain degree, because of the 

 action of frictional resistances which render necessary the introduc- 

 tion of a component of the gradient parallel to the motion of the air 

 and directed forward with it. The change of the vertical differ- 

 ence of pressure is to be considered as primarily a consequence of 

 the circumstances of the motion since the effectof thelattercan here 

 be conceived of as a mere diminution of the force of gravity. 12 



12 See the exponent of equation (28) where the influence of velocity opposes 

 that of gravity. — Abbe. 



