CROLVS ''CLIMATE AND TIMEP 815 



the importance of this agency in his presidential address before the 

 geographical section of the British Association, at its Dublin meeting, 

 in which he even went so far as to maintain that one part of the gen- 

 eral oceanic circulation takes place through the atmosphere ; though 

 in thus considering the aerial circulation to be telluric, or universal over 

 the whole surface of the earth, instead of hemispheric, or comparatively- 

 independent on opposite sides of the equator, this distinguished ex- 

 plorer seems to assume to be true that which is most emphatically con- 

 tradicted, not only by the almost innumerable observations collated by 

 Professor Coffin in the great work already referred to, but by those of 

 all other observers. It is true, though, that, as the transportation of 

 the vapor is accomiDlished by aerial currents, these currents indirectly 

 cause those marine currents formed by the precipitation of vapor. 2. 

 The earth's axial rotation, which is one of the two great causes of aerial 

 circulation, must exercise considerable effect on the marine currents, 

 though of course its operation is more seriously interfered with by the 

 inequalities of the terrestrial surface in the case of the earth's discon- 

 tinuous liquid enveloj)e, than in the universal gaseous one. Still, if, as 

 suggested by Guilleman, this force is adequate to influence the course 

 of rivers flowing to the north or south, it will vitiate any theory which 

 neglects it. Moreover, the actual effect of the earth's axial rotation 

 would be very likely to elude observation, as the currents so produced 

 would necessarily correspond approximately with the principal aerial 

 currents of the globe ; and hence the combined effect of the two causes 

 would be likely to be attributed to the most obvious one. The influ- 

 ence of axial rotation on the oceanic circulation may be determined 

 mathematically, however, and indeed such an investigation has already 

 been entered upon by Mr. Ferrel ; but his results are not generally 

 known. 



While both branches of the gravitation theory are summarily dealt 

 with, analogy with the second prime cause of aerial circulation (i. e., 

 difference in density, owing to differences in temperature and humid- 

 ity) would indicate that any difference in density, whether due to 

 difference in temperature or in saltness, would be sure to play a minor 

 part in the phenomenon of oceanic circulation- at least in some cases. 

 Therefore, of the five distinct agencies which probably cooperate in 

 disturbing the equilibrium of the ocean. Dr. Croll recognizes but one ; 

 and, singularly, in view of his usual method, he nowhere endeavors to 

 prove mathematically, or in any other conclusive manner, that the 

 impulse of the winds is adequate to produce the effects attributed to 

 it. The wind theory can not, therefore, be said to be demonstrated. 

 Nevertheless, the agreement between the winds and the marine cur- 

 rents is so striking that geographers and navigators are generally dis- 

 posed to adopt that theory. The late distinguished geographer of 

 Gotha, Dr. Petermanu, was one of the most prominent advocates of 

 the theory. 



