But it is quite admissible to accept the nebular hypothesis and the view that our globe attained its present form while still in a state of fluidity, and at the same time to maintain that our earth has long since reached its condition of complete solidification. And there are not a few facts which appear to lend support to such a conclusion. [Sidenote: SUPPOSED PROOFS OF LIQUID NUCLEUS.] If the rapid rate of increase in temperature which has been demonstrated to occur at so many parts of the earth's surface be maintained to the centre, then, as argued by David Forbes and Dana, it is difficult to conceive of our earth as being in any other condition than that of a liquid mass covered by a comparatively thin crust. The objection to this view, both upon geological and astronomical grounds, we have pointed out in the previous chapter. Before accepting as a demonstrated conclusion this notion of a constant increase of temperature from the surface to the centre of our globe, it may be well to re-examine the facts which are relied upon as proving it. That there is a general increase of temperature so far as we are able to go downwards in the earth's crust, there can, as we have seen, be no doubt whatever. Yet it may be well to bear in mind how very limited is the range of our observation on the subject. The deepest mines extend to little more than half-a-mile from the surface, and the deepest borings to little more than three-quarters of a mile, while the distance from the earth's surface to its centre is nearly 4,000 miles. We may well pause before we extend conclusions, derived from such very limited observations, to such enormous depths. But when we examine critically these observations themselves, we shall find equal grounds for caution in generalising from them. There is the greatest and most startling divergence in the results of the observations which have been made at different points at the earth's surface. Even when every allowance is made for errors of observation, these discrepancies still remain. In some places the increase of temperature as we go downwards is so rapid that it amounts to 1° Fahrenheit for every 20 feet in depth, while in other cases, in order to obtain the same increase in temperature of 1° Fahrenheit, we have to descend as much as 100 feet. Now if, as is so often assumed, this increase of temperature as we go downwards be due to our approach to incandescent masses forming the interior portions of the globe, it is difficult to understand why greater uniformity is not exhibited in the rate of increase in different areas. No difference in the conducting powers of the various rock-materials is sufficient to account for the fact that in some places the rate of increase in temperature in going downwards is no less than five times as great as it is in others. [Sidenote: VARIATIONS IN UNDERGROUND TEMPERATURES.]