248 LIGHT SCIENCE FOR LEISURE HOURS. 



equatorial water is lower than that of tropical water. 

 Now, it is unquestionably true that the effect of evapo- 

 ration is to increase the specific gravity of sea water ; 

 but it is equally true that the effect of the heat which 

 causes the evaporation is to diminish the specific 

 gravity. The point is considered in my essay entitled 

 'Is the Gulf Stream a Myth?' in the first series of 

 4 Light Science for Leisure Hours.' ' We recognise,' I 

 there say, ' 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.' And I 

 proceed to inquire which cause is likely to be the more 

 effective, arriving at the conclusion that the water is 

 made lighter. The case, indeed, appears to me to be 

 altogether different from that of the Mediterranean 

 Sea cited by Dr. Carpenter. In the Mediterranean we 

 have the same heating action as on the Atlantic in the 

 same latitudes, but not the same relatively enormous 

 quantity of water freely communicating with the region 

 so heated. We have, then, in the Mediterranean 

 evaporation as everywhere else, and evaporation to the 

 same degree, appreciably, as elsewhere in similar lati- 

 tudes ; but evaporation not compensated as in the open 

 Atlantic by the effects of free communication with 

 surrounding water. Hence we have in the Mediter- 

 ranean an increase of saltness ; in other words, an in- 

 crease of specific gravity. And precisely because this 

 increase takes place in the Mediterranean, whereas the 



