290 PHYSICAL GEOGRAPHY. 



wide fissures, capable of receiving large masses of water, were 

 frequent, and when the constitution of the atmosphere dif- 

 fered materially from what it now is, great changes of the 

 oceanic level may have taken place from variations in the 

 actual quantity of water : but in the present state of our 

 planet, there is no direct evidence whatsoever of a progres- 

 sive augmentation or diminution of the waters of the sea ; nor 

 is there any evidence of a progressive change in the mean 

 height of the barometer at the level of the sea. According 

 to Daussy and Antonio Nobile's experimental investigations, 

 an increase in the mean height of the barometer would 

 itself occasion a depression of the level of the sea. As, how- 

 ever, the mean pressure of the atmosphere is not the same in 

 all latitudes, owing to meteorological causes, such as the gene- 

 ral direction of the wind and the hygrometric state of the air, 

 the barometer alone would not afford certain evidence of varia- 

 tions in the general level of the ocean. The remarkable fact, 

 that several of the ports of the Mediterranean were repeatedly 

 left dry for several hours about the commencement of the pre- 

 sent century, appears to shew that, without any actual diminu- 

 tion of the general mass of water, or any general depression of 

 the oceanic surface, changes in the direction and strength of 

 currents may occasion a local retreat of the sea, and possibly 

 a permanent emersion of a small portion of coast ; but we 

 cannot be too cautious in the interpretation of our very 

 recently acquired knowledge of these complicated phenomena, 

 lest we should be led to attribute to one of the old " four 

 elements," water, that which really belongs to two others, 

 viz. the air and the earth. 



As the external form of continents, in the varied and deeply- 

 indented outline of their coasts, exercises a beneficial intiu- 



