42 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



Carpenter assumes. Further, he maintains that if difference 

 of specific gravity fail in accounting for the circulation of the 

 ocean in general, it fails in a more decided manner to explain 

 the Gibraltar current, because it is only the stratum of water 

 which rests above the level of the shallowest part of the 

 strait on each side that can exercise any influence in disturb- 

 ing equilibrium, and since the observed difference of density 

 between the Mediterranean and Atlantic within these limits 

 does not give a difference of level sufficient to cause move- 

 ment. 13 A, JVov. 1, 1871, 500. 



CROLL ON OCEAN CIRCULATION. 



Mr. Croll, in further discussion of the subject upon which 

 he and Dr. William P. Carpenter are at variance namely, 

 that of " ocean currents" remarks, in Nature, that the true 

 way of considering the matter is to regard the currents as 

 merely one grand system of circulation, produced, not by the 

 trade winds alone, but by the combined action of all the winds 

 capable of producing this action; and the effect upon the cur- 

 rents depends upon two circumstances, namely, the direction 

 of the prevailing winds and the conformation of the sea and 

 land. From this it results that the general system of winds 

 may sometimes produce a current directly opposite to the 

 prevailing wind blowing over the current. 



Taking into the account the result of the conformation of 

 the sea and land, Mr. Croll thinks, and he expects to show, 

 that all the principal currents of the globe, the Gibraltar cur- 

 rent not excepted, are moving in the exact direction in which 

 they ought to move, assuming the winds to be the sole im- 

 pelling cause. The influence of the rotation of the earth he 

 considers greatly overestimated, such rotation exercising no 

 influence in generating motion on the earth's surface ; but if 

 the body be already in motion, the rotation will deflect it to 

 the right in the northern hemisphere, and to the left in the 

 southern. 



Difference of specific gravity, as resulting from difference of 

 temperature between the equatorial and polar regions, might, 

 if sufficiently great, produce some such interchange of equa- 

 torial and polar water as Dr. Carpenter supposes ; but this 

 difference of temperature, in Mr. CrolPs opinion, could not 

 produce currents like the equatorial current and Gulf Stream 



