water at 4°c. They both should have known that sea water does not 

 have a point of maximum density at 4°c. like fresh water, and that 

 earlier explorers using the older method of hauling up water in an 

 insulated vessel had found bottom water with temperatures below 

 the freezing point of fresh water. 



It took a long time to get all the thermometers properly pro- 

 tected, and then the method was changed again. For the past sixty 

 years we have been using protected thermometers that turn upside 

 down when a catch on the water sampler is released by the 'mes- 

 senger' weight. This reversal separates a column of mercury pro- 

 portional to the temperature of the water and provides a tempera- 

 ture reading accurate to a hundredth of a degree. 



Unhappily, we have too few observations to tell us whether the 

 oceans as a whole are now getting warmer or colder. However, we 

 know a good deal about the seasonal changes, and in one or two 

 areas, such as the approaches to the Arctic, we have evidence that 

 the sea has become warmer in the past few decades. It is quite 

 remarkable how much we know about the large-scale features of 

 temperature distribution in the depths of the ocean as well as at 

 the surface, but we know much less about the fine structure and 

 the day-to-day changes. The interchange of heat and water vapor 

 between the atmosphere and ocean, together with the part played 

 by ocean currents in determining where most heat is going to be 

 fed into the atmosphere, is being studied in the new attack on the 

 sea. The surface temperature of the oceans increases from freezing 

 point in high latitudes to nearly 30°c. a little north of the Equator, 

 but the isotherms (lines of equal temperature) are not always 

 parallel to the lines of latitude. They are often bent toward the 

 poles by warm currents moving into higher latitudes, and toward 

 the Equator by cold currents moving away from the polar regions. 

 Because the temperature of the sea generally decreases with depth, 

 welling up from below tends to lower the surface temperature. 



Temperatures of the oceans generally decrease with depth be- 

 cause the lower layers are filled with water from higher latitudes. 

 A sea like the Mediterranean, on the other hand, largely shut off 

 from the ocean outside, is covered with a sun-warmed layer some 

 thirty to forty fathoms deep in summer. Only in such deep basins 

 shut off from the open ocean does the bottom temperature rise 

 above 2.5°c. Over most of the world ocean the bottom tempera- 

 ture is lower — only about i°c. or less. 



If the oceans were evaporated and all the salt spread evenly over 

 the earth, there would be a salt layer about two hundred feet thick. 

 We do not know just how and when the ocean acquired it all — 

 how much was formed at the same time as the oceans themselves, 

 and how it has been leached out of the land. Perhaps the salts 

 accumulated at a great rate to begin with, or perhaps as a gradual 

 process. The minerals brought down by the rivers today look 

 rather like the less soluble residues. The total annual addition of 

 salt to the ocean is now less than a ten millionth of what is already 

 there. 



Robert Boyle was the first to make a detailed study of the salt 

 content of the oceans. In 1673, when he published his Observations 

 and Experiments about the Saltness of the Sea, he realized that many 

 observations would have to be made in all parts of the world before 

 anyone could generalize about the amount of salt in the ocean. He 



TRACTS 



Confiding of 



OBSERVATIONS 



About the 



SALTNESS of the S E Aj 



Ati Account of a 



St A tic At HTCROS COPS 



Aad its USES: 



Together with »n APPENDIX 



".», — .. -about, the,. ,^^^^„„ 



FOR.b?^of t^e AI^tS,MOISTaR.E 5 



AFRAGMfi^t ibout the] 



NATllRALaad PRETERNAT|iRAL 



STATE of »ODIES?i 



By (he Honourable ROBERT B Or LE. 



W] To all which is premis'd 



A SCEPTrCAL DIALOGUE 

 About (he POSITIVE or PRIVATIVE 

 NATURE of COLD: 



With fome Experiment* of Mr. BOrVS rcferr'd 

 to in chat Oifcouife. 



By a Member of the ^Or/4 1. SOCIETY. 



Luiion, Pijnted by E.Flefltr for R.DMif Bookfellct 

 in (IshrJ M DC LXXIir 



The title page of Robert Boyle's tract on 

 "saltness of the sea." In 1673 Boyle knew 

 that the amount of concentration of salt 

 in any region of the sea depended on the 

 balance between rainfall and evaporation. 



179 



