8 14 THE POPULAR SCIENCE MONTHLY. 



the ocean to maintain its level, and naturally select the path of least 

 resistance. Hence they are usually undercurrents. 



The general agreement between the systems of winds and of oceanic 

 circulation is shown on a chart. It may be mentioned that the direc- 

 tion of the winds, as laid down on this chart, does not precisely corre- 

 spond with the direction determined by the late Professor Coffin, and 

 indicated on the charts in his cyclopean work, " The Winds of the 

 Globe," recently published by the Smithsonian Institution ; and it is 

 quite probable that more exhaustive observations will show that there 

 are inaccuracies of as great magnitude in the representation of oceanic 

 currents. It should be borne in mind, however, that any minor dis- 

 crepancies between the two systems do not militate against the theory, 

 unless it can be shown that they are not such as would be produced 

 by the conformation of the coasts ; for the wind system, as a cause of 

 oceanic circulation, is modified by this important and frequently an- 

 tagonistic factor. 



In seeking for demonstrative evidence of the correctness of the 

 wind theory. Dr. Croll seems to fall into an error which he has repeat- 

 edly had occasion to point out in others. Thus, in discussing one of 

 Dr. Carpenter's sections, he shows, from Professor Muncke's coeffi- 

 cients of the density of sea-water, at various temperatures, that the 

 surface of the Atlantic is, at north latitude 23° 10', two feet six inches, 

 and at north latitude 37° 54', fully three feet six inches higher than at 

 the equator, on account of the greater thickness at these points of the 

 upper layer of warm water. It is urged that " gravitation never could 

 have caused the ocean to assume this form," and hence that " gravita- 

 tion can no more cause the surface-water of the Atlantic to flow toward 

 the Arctic regions than it can compel the waters of the Gulf of Mexico 

 up the Mississippi into the Missouri." Now, by Dr. Croll's own show- 

 ing, it is gravity alone that causes the surface of the Atlantic to assume 

 that form — that is, if that form is actually assumed ; a fact not deter- 

 mined by measurements. It is merely calculated from the data given 

 to what height above the level at the equator the waters of the North 

 Atlantic would have to be raised in order to maintain a condition of 

 static equilibrium ; it is 'not shown that the waters are so elevated, but 

 only that they would have to be in order that the influence of gravita- 

 tion, in producing oceanic currents, should be eliminated. How the 

 warm water, on which this supposed configuration depends, was con- 

 veyed thither, is a dynamical and not a statical problem. 



Moreover, two important elements in the problem of oceanic circu- 

 lation are disregarded by Dr. Croll : 1. The water borne from equa- 

 torial to polar regions by aerial currents, in the form of aqueous vapor, 

 must exercise a powerful influence not only on the earth's temperature 

 but also upon marine currents ; for, as shown by Lieutenant Maury, it 

 is sufficient to permanently render the Arctic seas much less saline than 

 those of tropical regions. Sir Wy ville Thomson strongly insisted upon 



