Page:Popular Science Monthly Volume 16.djvu/849

Rh 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 general oceanic circulation takes place through the atmosphere; though in thus considering the aërial 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 explorer seems to assume to be true that which is most emphatically contradicted, 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 accomplished by aërial 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 aërial 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 discontinuous liquid envelope, 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 aërial 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 influence 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 aërial circulation (i. e., difference in density, owing to differences in temperature and humidity) 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 coöperate 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 currents is so striking that geographers and navigators are generally disposed to adopt that theory. The late distinguished geographer of Gotha, Dr. Petermann, was one of the most prominent advocates of the theory.