Dr. CroWs Theory of Glacial and Warm Periods. 75 



can only add weight to the polar column by increasing its 

 height, and therefore reducing the difference between it and 

 the column at the equator. "Suppose then," says Dr. Croll, 

 " that the weight of the polar column exceeds that of 

 the equatorial column by 2 feet of water, the difference of 

 level between the two columns would then amount to only 

 2 feet 6 inches. This would give a force of ^ that of 

 gravity, a force which does not much exceed a grain on a 

 ton of water." This would be the measure of the force 

 tending to push the water at the pole towards the equator. 

 But, as the experiments of Dubuat have shown, water 

 will not flow on a slope of 1 in 1,000,000, that is to say, 

 when the force of gravity acting on it is thirteen times 

 greater than in the case just cited. This seems clearly to 

 show that the cause appealed to by Dr. Carpenter is quite 

 inadequate to explain the origin of ocean currents. 



Dr. Croll has several minor polemics with his opponent 

 and his supporters, but this complete answer is sufficient, 

 and it seems to me to put out of court all theories of ocean 

 circulation depending on gravity for their explanation. 



After criticizing the theories which attribute the flow of 

 ocean currents to the rotation of the earth and to gravity, 

 and, in my view, showing them to be impossible, Dr. Croll 

 goes on to argue that the great ocean currents are the result 

 of the pressure and traction of winds, a view in which he was 

 forestalled in a valuable paper published as far back as 

 185 1, by Alderman Thomas Hopkins, in the Memoirs of 

 this Society, which he would no doubt have quoted if he 

 had known of it, for he is a model of fairness as well as 

 lucidity. Dr. Croll has elaborated many arguments to show 

 that winds are competent to create ocean currents, and that 

 the observed facts can be explained, and explained most 

 satisfactorily, by their agency, and there can be small doubt 

 that here again he is substantially right. But while con- 

 ceding that the winds are the main cause of ocean currents 



