CHAP, viii.] THE GULF-STREAM. 365 



It is not easy to understand Captain Maury's view. 

 He traces all ocean currents to differences in specific 

 gravity. He says : " If we except the tides, and the 

 partial currents of the sea, such as those that may be 

 created by the wind, we may lay it down as a rule 

 that all the currents of the ocean owe their origin to 

 the differences of specific gravity between sea-water 

 at one place and sea-water at another ; for wherever 

 there is such a difference, whether it be owing to dif- 

 ference of temperature or to difference of saltness, 

 &c., it is a difference that disturbs equilibrium, and 

 currents are the consequence." 1 These differences 

 in specific gravity he attributes to two principal 

 causes ; differences in temperature, and excess of salts 

 produced by evaporation. Captain Maury explains 

 his views as to the first of these causes by an illustra- 

 tion. " Let us now suppose that all the water within 

 the tropics to the depth of one hundred fathoms sud- 

 denly becomes oil. The aqueous equilibrium of the 

 planet would thereby be disturbed, and a general 

 system of currents and counter-currents would be 

 immediately commenced, the oil in an unbroken sheet 

 on the surface running towards the poles, and the 

 water as an under-current towards the equator. The 

 oil is supposed, as it reaches the polar basin, to be re- 

 converted into water, and the water to become oil as 

 it crosses Cancer and Capricorn, rising to the surface 

 in intertropical regions, and returning as before." 

 " Now, do not the cold water of the north, and the 

 warm water of the gulf made specifically lighter by 

 tropical heat, and which we see actually presenting 



1 The Physical Geography of the Sea, and its Meteorology. By 

 M. T. Maury, LL.D. 



