Mr. J. Croll on the Physical Cause of Ocean-currents. 243 



but, on the other hand, if the work of gravity falls short of 

 the work of the resistances (and Dr. Carpenter has nowhere 

 proved that it does not), the motion which he supposes is not 

 only not a physical necessity, but is actually a physical impos- 

 sibility. 



Dr. Carpenter states that his doctrine of a General Oceanic 

 Circulation has been accepted as valid by some of the most dis- 

 tinguished mathematicians and physicists of this country. This 

 is, no doubt, true; but I cannot help thinking that those emi- 

 nent physicists who have given a general assent to the correct- 

 ness of his theory have done so without giving it special consi- 

 deration — and that when they come to examine the question 

 more minutely they will be satisfied that the forces resulting 

 from difference of density, whether that difference be caused 

 by difference of temperature or by difference in saltness, is so 

 infinitesimal as to be wholly insufficient to produce the great 

 currents of the ocean. With the exception of Dr. Colding, of 

 Copenhagen*, I am not aware that any physicist holding the 

 gravitation theory has attempted to determine the absolute 

 amount of the forces acting on the water so as to produce motion. 



Dr. Carpenter's experiment. — True, Dr. Carpenter has exhibited 

 an experiment to show the motion of the water. But I presume 

 his experiment was intended rather to illustrate the way in which 

 the circulation of the ocean, according to his theory, takes place, 

 than to prove that it actually does take place. At any rate all 

 that can be claimed for the experiment is the proof that water 



* Dr. Colding, of Copenhagen, in a memoir lately published (Om 

 Stromningsforholdene i almindelige Ledninger og i Havet, 18/0), has de- 

 termined with much labour and skill the influence of difference of specific 

 gravity and of the earth's rotation as causes of the Gulf-stream. The fol- 

 lowing are some of the conclusions at which he has arrived. Between 

 Bernini and St. Augustine, the only motive power he considers is differ- 

 ence of level, which he estimates to be 6 feet. From St. Augustine to New 

 York Bay the stream is propelled by the rotation of the earth, the force of 

 which is' equal to that of a slope of 9 or 10 feet. From New York Bay 

 to Europe it is propelled east by rotation up a slope of about 1 foot. Near 

 Europe the current divides into two branches. One, under the influence 

 of the diminished force of rotation, goes south-east to the coast of Africa; 

 the other goes along the British coast, and is turned north by the direction 

 of the coast, rotation causing it to rise from left to right about 1| foot. The 

 estimated force of rotation exercised on the Gulf- stream from St. Augustine 

 to lat. 60° N. he considers to be equal to that of a difference of level of 

 25 feet. He has in like manner shown the influence of difference of level 

 and rotation on the return current from the Arctic regions to the Gulf of 

 Mexico. 



Dr. Colding appears, however, to leave out of account molecular resist- 

 ance to motion. He says, "we do not know at present if the molecules 

 of water or air move without resistance." But, for reasons which will 

 come under our consideration, Dr. Colding appears to have greatly over- 

 estimated the influence of gravity in the production of motion. 



B,2 



