146 LIGHT SCIENCE FOR LEISURE HOURS. 



oceanic circulation. If a long trough be divided into 

 two compartments, and we iill one with oil and the 

 other with water, and then remove the dividing plate, 

 we shall see the oil rushing over the water at one end 

 of the trough, and the water rushing under the oil at 

 the other. And if we farther conceive that oil is con- 

 tinually being added at that end of the trough origi- 

 nally filled with oil, while water is continually added 

 to the other, it is clear that the system of currents 

 would continue in action : that is, there would be a 

 continual flow of oil in one direction along the surface 

 of the water, and of water in the contrary direction 

 underneath the oil. 



Bat Sir John Herschel maintains that no sach 

 effects as Maury describes could follow the action .of 

 the sun's heat upon the equatorial waters. He argues 

 thus: Granting that these waters become lighter and 

 expand in volume, yet they can only move upward, 

 downward, or sideways. There can be nothing to 

 cause either of the first two forms of motion ; and as 

 for motion sideways, it can only result from the 

 gradual slope caused by the bulging of the equatorial 

 waters. He proceeds to show that this slope is so 

 slight that we cannot look upon it as competent to 

 form any sensible current from the equatorial toward 

 the polar seas. And even if it could, he says, the 

 water thus flowing off would have an eastward instead 

 of a westward motion, precisely as the counter-trade- 



