JOLY'S ESTIMATE 21 



contracted period will yield to a little stretching, Pro- 

 fessor Joly's, on the other hand, may take some paring. 

 His argument, broadly stated, is as follows. The ocean 

 consisted at first of fresh water ; it is now salt, and its 

 saltness is due to the dissolved matter that is constantly 

 being carried into it by rivers. If, then, we know the 

 quantity of salt which the rivers bring down each year 

 into the sea, it is easy to calculate how many years they 

 have taken to supp]y the sea with all the salt it at 

 present contains. For -several reasons it is found neces- 

 sary to restrict attention to one only of the elements 

 contained in sea salt : this is sodium. The quantity of 

 sodium delivered to the sea every year by rivers is about 

 160,000,000 tons ; but the quantity of sodium which the 

 sea contains is at least ninety millions of times greater 

 than this. The period during which rivers have been 

 carrying sodium into the sea must therefore be about 

 ninety millions of years. Nothing could be simpler; 

 there is no serious flaw in the method, and Professor 

 Joly's treatment of the subject is admirable in every way ; 

 but of course in calculations such as this everything 

 depends on the accuracy of the data, which we may there- 

 fore proceed to discuss. Professor Joly's estimate of the 

 amount of sodium in the ocean may be accepted as 

 sufficiently near the truth for all practical purposes. We 

 may therefore pass on to the other factor, the annual 

 contribution of sodium by river water. Here there is 

 more room for error. Two quantities must be ascer- 

 tained : one the quantity of water which the rivers of the 

 world carry into the sea, the other the quantity or pro- 

 portion of sodium present in this water. The total 

 volume of water discharged by rivers into the ocean is 

 estimated by Sir John Murray as 6,524 cubic miles. The 

 estimate being based on observations of thirty-three great 

 rivers, although only approximate, is perhaps sufficiently 



