130 LIGHT SCIENCE FOR LEISURE HOURS. 



against the wind, after leaving the Gulf of Mexico. 

 But experience is wholly opposed to this view. The 

 most energetic currents are quickly dispersed when 

 they reach a wide expanse of still water. For example, 

 the Niagara below the falls is an immense and rapid 

 river. Yet when it reaches Lake Ontario, * instead of 

 preserving its character as a distinct and well-defined 

 stream for several hundred miles, it spreads itself out, 

 and its waters are immediately lost in those of the 

 lake.' Here, again, the question asked by Maury 

 bears pertinently on the subject we are considering. 

 ( Why,' he says, ( should not the Gulf Stream do the 

 same ? It gradually enlarges itself, it is true ; but, 

 instead of mingling with the ocean by broad spread- 

 ing, as the immense rivers descending into the northern 

 lakes do, its waters, like a stream of oil in the ocean, 

 preserve a distinctive character for more than three 

 thousand miles.' 



The only other theory which has been considered in 

 recent times to account satisfactorily for all the features 

 of the Gulf- Stream mechanism was put forward, we 

 believe, by Captain Maury. In this theory, the 

 motive power of the whole system of oceanic circula- 

 tion is held to be the action of the sun's heat upon 

 the waters of the sea. We recognise two contrary 

 effects as the immediate results of the sun's action. In 

 the first place, by warming the equatorial waters, it 

 tends to make them lighter ; in the second place, by 

 causing evaporation, it renders them salter, and so 

 tends to make them heavier. We have to enquire 



