ON THE SOURCES OF HEAT WHICH INFLUENCE CLIMATE. 403 



gravity would be obtained greater than that which, from astronomical data, we 

 know our globe to possess ; and as their density would go on increasing as we 

 continue to recede from the surface, he was led to conjecture that the terrestrial 

 sphere had a hollow centre. He further came to the striking result, that air will, 

 at considerable depths, from its higher elasticity, become heavier than water ; if 

 we then grant that water at the greatest depths of the ocean contains air in 

 solution, as we find it to do on the surface, Leslie's theory will tend to the 

 conclusion that the ocean rolls on a bed of air. He adopts this view, and 

 endeavours to connect it with some of the phenomena of volcanoes. 



To suppose the seats of volcanic fire at the depths Leslie has done, would 

 bring us to internal pressures on parts of the crust of our globe exceeding, on 

 the most moderate scale of computation, 20,000 atmospheres. This appears to 

 me to render it extremely improbable that the seats of volcanoes are deep in the 

 earth's surface. In practice we know of no pressures equal to 100 atmospheres ; 

 we also find how difficult it is to construct metallic vessels able to resist the force 

 of a few atmospheres ; the materials of the crust of the globe are, moreover, of 

 a composition not well calculated for the resistance of pressure. Yet there is no 

 doubt, from the data furnished us by volcanic eruptions, that during such events 

 a small portion at least of the earth's crust must be capable of overcoming a con- 

 siderable force. 



We have only to suppose the centre of volcanic fire to be 2,100 feet below the 

 orifice of a crater in activity, to have there an internal pressure of 100 atmos- 

 pheres- There are on record many eruptions where lava was discharged in large 

 quantities that must have proceeded from depths greater than this ; but I am 

 much inclined to assign to the centres of volcanic eruptions a position as near the 

 earth's surface as is consistent with recorded phenomena. I am very hostile to 

 connecting them with the incandescent matter occupying the earth's centre, 

 chiefly from the consideration that they would then communicate when in an 

 active state an enormous force over every point of the interior of the inclosing 

 crust ; there are other reasons that render such an hypothesis extremely im- 

 probable. 



Fluids, wherever situated, must be influenced by the all-pervading law of 

 gravitation. The attraction of the sun and moon will develope tidal waves in 

 every considerable mass of fluid matter ; hence, in the event of a crater com- 

 municating with such a mass, we should look for its discharge being varied by 

 tides ; from this circumstance I think it can be proved that they are not in 

 connexion with any great quantity of fluid. 



The existence of internal tides bears on another question. If the solid surface 

 of the earth be comparatively only a thin crust, Ave should have a powerful 

 diurnal tide, which would occasion violent dislocations in the strata. This view 



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