PROCEEDINGS OF THE POLYTECHNIC! ASSOCIATION. 729 



tides are known to be purely the effect of the force of gravity, be- 

 tween the sun and moon, and the earth, and the heat of the sun's rays, 

 by changing the specific gravity of the water of the ocean in the dif- 

 ferent seasons, produces annual modifications of tlie ocean currents, 

 but the force by w^hicli a central orb could produce such various 

 and irregular changes as are attributed to it, in the form of a solid 

 globe, without manifesting itself in a corresponding manner upon 

 the liquid ocean, where even the slightest air-currents produce per- 

 ceptible movements, must be widely different from any force now 

 known to us. 



While admitting that the interior heat of the earth increases 

 with the depth, Mr. Wood argues, with some apparent force, 

 that fluidity is not a necessary consequence, because the enormous 

 pressure produced by the weight of the masses above, would 

 cause solidification at very high temperatures. The laws by 

 which solidification would take place under excessive pressure are 

 not, perhaps, well enough understood to enable us to judge of 

 this with certainty, but such experimental knowledge as we pos- 

 sess points in the opposite direction. The ocean always freezes 

 at the surface, and not at the bottom; and Tyndall has shown that 

 ice is converted to water by pressure at a temperature below the 

 freezing point. In other words, pressure lowers instead of raising 

 the temperature of the freezing point. We are sometimes made 

 aware of the enormous force with which this pressure is resisted, 

 when the temperature of water in confined vessels becomes at 

 length so reduced that crystalization or freezing takes place. 

 Vessels of immense streng^th are burst in this way, rocks are split 

 asunder, &c. I am not aware whether volcanic eruptions and 

 earthquakes have been attributed to this property of expansion in 

 passing from fluidity to solidity, but it seems to me worthy of 

 consideration. The gradual cooling under pressure of a thin 

 layer of liquid next to the outside crust would, at the proper 

 moment, result in an instantaneous outward pressure, and the 

 crust would probably yield at its -weakest points. Mountain 

 regions having yielded to the force of lateral pressure would be 

 likely to yield most readily to this action, for it is here that the 

 least resistance is offered to pressure either outward or inward. 



But the final and culminatino- argument of Mr. W^oods is that 

 gravity is the cause, not only of the internal heat of the earth, 

 but of the sun's heat. He adduces the observations of navigators, 

 to the effect that the ocean is a few degrees warmer at considera- 



