1893-94-] The Interior of the Earth. 101 



If we assume that the rate of increase in temperature of 

 1° for every 51 feet is maintained, on arriving at a depth 

 of a little under 2 miles the rocks would be as hot as boiling 

 water; at a depth of 27 miles the temperature would be 

 as high as 2794° F., which would be sufficient to melt steel; 

 and at 40 miles, which is really only a little way down, we 

 should have a temperature higher than any we could produce 

 at the surface. 



It is highly probable that at these great depths the ma- 

 terials, though at an intense heat, are not in a state of fusion, 

 but are more or less pasty or even solid, owing to the enor- 

 mous superincumbent pressure. It is well known, in the case 

 of many solid substances with which we are acquainted, that 

 pressure has the effect of raising the melting-point to a small 

 extent. The direct effect of pressure on the fusing-point of 

 rock has yet, I believe, to be studied, and it is likely that 

 different ratios would be found for different materials. 



It has been supposed that the ratios of pressure and tem- 

 perature within the earth are such, that below a certain depth 

 the pressure is only just sufficient to prevent the liquefaction 

 of the rocks by the temperature proper to that depth, so that 

 any local cause diminishing the pressure would allow of lique- 

 faction. 



Again, it is possible that the ratios of the increase of pres- 

 sure and temperature are such that, at a certain depth, the 

 pressure is insufficient to prevent liquefaction at all, and con- 

 sequently, that a certain thickness of the earth's mass is 

 liquid ; but that, at a still greater depth, the increased pressure 

 overcomes the influence of heat, and keeps the rocks solid in 

 spite of increased temperature. It is also possible that be- 

 yond a certain depth, which perhaps may not be so very 

 great, there is no further rise in temperature. 



Whether the interior be to a great extent fluid or solid, we 

 may safely admit that it has a very high temperature, and in 

 that case it must be continually parting with it by radiation. 

 The cooling has proceeded so far now that the heat, if any, 

 passing into space through the outer crust at the present time 

 is so small in amount that it does not appreciably affect the 

 zone which lies above the depth of 50 feet from the surface, 

 the " limit of seasonal variation." 



