OCEAN TEMPERATURES AND CURRENTS. 239 



already referred to, explains them as follows. If we 

 watch a ship passing along through the water, we will 

 notice that it seems to depress the water at its bows, 

 thereby causing currents to set in from the higher water 

 on both sides of these depressions on either side of the 

 ship. In like manner the sun in passing around the 

 globe compresses the waters around the equator, and 

 hence starts a current from the higher waters of the 

 northern and southern regions. This was in the age 

 when perpetual motion was the "craze," and he found 

 his source of perpetual motion in the tropics. It is 

 certainly very strange that men allow their imaginations 

 to run away with them to such an extent. 



Up to the year 1700, we find no satisfactory explana- 

 tion of these currents. At that time it was believed 

 that the strong evaporation of water at the equator pro- 

 duced them. It can be easily shown, however, that such 

 variations will only produce local currents, and that the 

 effects produced by the heat of the sun in this way are 

 not sufficiently far-reaching in their power to account 

 for our great ocean currents. Then came Franklin with 

 his proposition that they were initiated in their course 

 by the constant action of the trade winds, which reason- 

 ing we shall see, later, has considerable force. Later on, 

 in this century, we find Maury advocating the differ- 

 ences in specific gravity of the waters as the main cause. 

 He believed that the cold waters at the poles sank, and 

 then moved towards the equator, being replaced by the 

 warm waters which came from the tropics, and were in 

 turn cooled off. There is a kernel of truth in this idea, 

 but it does not in any way explain the origin of the 

 motion. The sinking of cold water and the rising of 



