/ 



PRESIDENTS ADDRESS. 621 



of interest.*" This water is considered by M. Bertram Blount to 

 be entirely plutonic in its origin, and, while it contains consider- 

 able proportions of calcium and magnesium sulphates, is remark- 

 able in being quite free from chlorides. The chlorides are 

 amongst the most volatile of the so-called fixed salts, and hence 

 would be the last to condense during the genesis of the earth. 

 Professor Joly has based calculations regarding the age of the 

 earth upon the estimated rate at which chlorides are now being 

 carried by water from the land to the ocean ; but consideration 

 of the probabilities as to the condensation of such substances 

 during the earlier part of the earth's history and the consequent 

 enormous initial saltness of ihe sea, indicates how very unsafe 

 are the conclusions arrived at from evidence of the kind.f 



If the process of land denudation now going on lasts for a 

 sufficiently long period the time will come when the whole surface 

 of the globe will be covered with water, and, if the solid matter 

 is uniformly spread over the ocean floor, the universal ocean will 

 have a depth of about 1,700 fathoms. The mean present depth 

 of the sea is about 2,500 fathoms, or almost three miles, so that 

 the mass of land above sea level is sufficient in bulk, if thus 

 spread out, to reduce the depth over the whole surface by only 

 some 800 fathoms. As a matter of fact the submerged valleys 

 beneath the surface of the sea are sufficient in size to contain 

 quite three times as much bulk as there is of dry land above 

 sea level. 



The greatest known depth is about 27,700 feet, or 5| miles. 

 At such enormous depths as this the pressure of the water is very 

 great. A column of fresh water one mile in depth exerts a 

 pressure of 2,288 lbs. per square inch, whilst the same depth of 

 the heavier sea water has a pressure of 2,347 lbs. For a depth 

 then of 5 J miles we have the prodigious pressure of 12,320 lbs., 

 or 5^ tons per square inch. 



* The Analyst, xxx.,o(J7. 



t See Prof. G. H. Darwin's Presidential Address to the British Association. 



Nature, August 81st, 1905 ; Chemical News, Ixxxix. ]3 (1904). 



