SHOPwT MEMOIRS ON METEOEOLOGICAL SUBJECTS. 450 



currents, and tliat tbe level surfaces of these cnrrents must necessarily 

 Lave an inclination toward tbe common boundary, quite in accordance 

 with what takes place in marine currents ; hence it further follows that 

 since the magnitude of the gap formed between the level surfaces of 

 the two currents is dependent upon the velocity of these cnrrents, any 

 chance stoppage of one of these currents will cause it to impinge against 

 the other, either from the NW. or the SE., thus producing an eddy 

 which moves "against the sun." It is evident that so long as the cur- 

 rent in question continues its motion, the force of the earth's rotation 

 maintains equilibrium with that force which corresponds to the inclina- 

 tion of the level surfaces toward the dividing surface between the two 

 currents ; but, as soon as the motion is diminished, the force of gravity 

 gets the upper hand, and an eddying movement is inevitable. It must 

 now be considered, however, that the density is usually different for the 

 two currents under consideration, since the polar air-current is generally 

 of greater specific gravity than the equatorial current, and the relation 

 between the two air-currents is therefore, in all respects, quite in 

 harmony with the relation which I have pointed out between the Gulf 

 Stream and the polar current along the coast of America. Since, namely, 

 the pressure in the upper part of the atmosphere for the two currents 

 cannot be very different, the pressure in the lower part of the atmos- 

 phere must be different for air-currents of different specific gravities; 

 and since two essentially different pressures cannot exist by the side of 

 each other, the consequence is that the current having the greater spe- 

 cific gravity must flow sidewise into the current having the less specific 

 gravity, in proportion to the excess of pressure. Since now, as has been 

 said, the polar current usually has the greater specific gravity of th^. 

 two currents, it follows that, in proportion to the excess of pressure of 

 the polar current above the equatorial current running east of the same, 

 under otherwise undisturbed atmospheric conditions, a considerable 

 portion of cold air will rush from the northwest into the equatorial cur- 

 rent, whose air-pressure is thereby increased as far as the cold air pene- 

 trates. It is, however, at the same time, evident that the cold air which 

 thus penetrates will, as soon as it penetrates into the warm air, which 

 is full of aqueous vapors, produce a condensation of these vapors, from 

 which a cold rain from the northwest or west will result. It must also 

 be evident from the foregoing that under disturbed atmospheric condi- 

 tions eddying winds may likewise be formed, which, under extraordinary 

 circumstances, may increase to tornadoes or hurricanes, with all the 

 corresponding natural phenomena. 



We have now considered the natural conditions that must present 

 themselves upon the west side of an equatorial current when a polar 

 current runs along it toward a southwesterly direction, and we will now 

 proceed to examine what must be the conditions upon the east side of 

 such an equatorial current when a polar current runs alongside of it in 

 an opposite direction toward the southwest. In the first place, it is 



