PHYSICAL GEOGRAPHY. 303 



Tho depression and elevation of the solid or fluid parts of 

 the Earth phenomena which are so opposite in their action, 

 that the effect of elevation in one part is to produce an 

 apparent depression in another, are the causes of all the 

 changes which occur in the configuration of continents. In 

 a work of this general character, and in an impartial exposi- 

 tion of the phenomena of nature, we must not overlook the 

 possibility of a diminution of the quantity of water, and a 

 constant depression of the level of seas. There can scarcely 

 be a doubt that at the period when the temperature of the 

 surface of the earth was higher, when the waters were 

 enclosed in larger and deeper fissures, and when the atmo- 

 sphere possessed a totally different character from what it 

 does at present, great changes must have occurred in the 

 level of seas, depending upon the increase and decrease of 

 the liquid parts of the earth's surface. But in the actual 

 condition of our planet, there is no direct evidence of a real 

 continuous increase or decrease of the sea, and we have no 

 proof of any gradual change in its level at certain definite 

 points of observation, as indicated by the mean range of 

 the barometer. According to experiments made by Daussy 

 and Antonio Nobile, an increase in the height of the baro- 

 meter would in itself be attended by a depression in the level 

 of the sea. But as the mean pressure of the atmosphere at 

 the level of the sea is not the same at all latitudes, owing to 

 meteorological causes depending upon the direction of the 

 wind and varying degrees of moisture, the barometer alone 

 cannot afford a certain evidence of the general change of 

 level in the ocean. The remarkable fact that some of the 

 ports in the Mediterranean were repeatedly left dry during 

 several hours at the beginning of this century, appears to 

 show that currents may, by changes occurring in their 

 direction and force, occasion a local retreat of the sea, and a 

 permanent drying of a small portion of the shore, without 

 being followed by any actual diminution of water, or any 

 permanent depression of the ocean. We must, however, be 

 very cautious in applying the knowledge which we have 

 lately arrived at, regarding these involved phenomena, since 

 we might otherwise be led to ascribe to water, as the elder 

 element, what ought to be referred to the two other elements 

 earth and air. 



