Dr. Croll' s Theory of Glacial and Warm Periods. 7 1 



already in motion, no matter in what direction the motion 

 may be, rotation will deflect it to the right in the northern 

 hemisphere and to the left in the southern hemisphere, as 

 has been shown by Mr. Ferrel {Nature, V., 201.) 



Putting aside the earth's rotation as an impossible cause 

 of ocean currents, we will now turn to the theory advocated 

 in different ways by Maury and Carpenter, that they are 

 due to gravitation. Maury urges that " all the currents of 

 the ocean owe their origin to the difference of specific 

 gravity between sea water at one place and sea water at 

 another ; for wherever there is such a difference, whether 

 it is owing to a difference of temperature or a differ- 

 ence of saltness, etc., it is a difference that disturbs equi- 

 librium and causes currents." 



Dr. Croll has no difficulty in putting Maury upon the 

 horns of a dilemma in regard to this double cause of density, 

 for while he urges in one place that the great ocean currents 

 are due to the high temperature of the equatorial regions 

 which make their waters less dense than those of the poles, 

 he urges in another that these same currents arise because 

 the equatorial waters, in consequence of their greater salt- 

 ness, are more dense than the waters of the polar regions ; 

 and Croll says, with justice, that these two causes neutralise 

 each other, and cannot be combined to produce a common 

 result. Not only so, but according to Maury himself, the 

 ocean, as far as 40° N. and S. of the equator, does not differ 

 in density ; that is to say, it does not differ in density w r here 

 the greatest of ocean currents takes it rise. 



But Dr. Croll is not content with creating this dilemma. 

 According to a very simple law of hydrostatics, if a sheet of 

 water is of uniform temperature and composition, and there- 

 fore of uniform specific gravity, its surface will be a perfectly 

 horizontal plane. If the composition remains the same, but 

 one portion of it is warmer than another it will requiie a 

 greater depth of water to make a column of it of the same 



