58 



done in Sir James Ross's Antarctic voyages, but it is now 

 known that the results then obtained were falsified by the 

 pressure of the water at great depths on the thermometers, 

 which caused the readings to be too high. The thermometers 

 are now " protected" in a way that gets rid of this source of 

 error ; and they show the water at great depths in the Atlantic 

 to be of a temperature not much exceeding the freezing point 

 of fresh water. This cold water must have come from the Polar 

 seas. There is a superficial current, or rather drift, from the 

 Equatorial to the Polar seas, and a return drift from the Poles 

 to the Equator at a great depth. This is due to the heating 

 of the Equatorial surface water by the sun, so as to expand it 

 and make it lighter ; it consequently flows off towards the 

 Poles, where it is cooled, contracts, becomes heavier, sinks 

 down, and flows as an under current back to the seas whence 

 it came. 



In the Mediterranean everything is different. The waters 

 of the Mediterranean are nowhere below' about 50 of Fahren- 

 heit's thermometer, even at depths which in the Atlantic are 

 almost icy cold. This shows that the cold bottom waters of 

 the Atlantic do not enter the Mediterranean. Unlike the At- 

 lantic, the bottom waters of the Mediterranean contain more 

 salt than the surface waters. A current, flows out of the At- 

 lantic through the Straits of Gibraltar into the Mediterranean ; 

 and it was ascertained by means of a" current drag," in- 

 vented for the purpose, that there is an under-current in the 

 opposite direction, from the Mediterranean into the Atlantic. 

 The explanation of these facts is as follows : — The Mediter- 

 ranean loses more water by evaporation than it receives from 

 rain and rivers, and the surface current flows in at the Straits 

 of Gibraltar to supply the deficiency. The surface water has 

 its salt in some degree concentrated, in consequence of the 

 evaporation, and thus it becomes heavier and sinks to the 

 bottom. Were the Straits of Gibraltar a mere river, the effect 



