Penny Blood: Serial Sensation and Working-Class Entertainment — page 290: what you’re looking at

Machine-transcribed from the original scan — historical spelling and the odd misread are preserved.

CH. XXII.] SUMMARY. 273 and suppose (which is of course impossible) that the crust had shrunk down upon it without becoming either thickened or wrinkled by the process. The position which the surface would have occupied under this impossible supposition, gives us a definite level, at a definite distance from the centre of the earth, and this level we call the ‘upper datum level.’ Simi- larly, the position which the under side of the crust would occupy, we call the ‘lower datum level.’ To these levels we refer the elevations and depressions (if any) of the surface, caused by the crumpling action; and we can express their amount by saying how deep a layer of material the elevations would form, if they were levelled down, and spread out upon our upper datum level. Now if we assume that the earth 1s solid, 2.e. that there is no fluid substratum beneath the crust, it is clear that all the inequalities which compression could produce would be of the nature of elevations above the upper datum level. There could be no depressions. Moreover the bottom of the ocean basins would either coincide with this datum level, or else they would be the parts of the disturbed surface least raised above it. Assuming the former to be the case, we find, upon an estimate as near as we see our way to form, that the whole of the existing inequalities of the surface, if levelled down and spread out, would form a layer of from 9500 to 13000 feet thick over the whole earth. This estimate is not likely to be too large, but rather the other way. VI. Our next step is to enquire, what the amount of the inequalities would be, if they had been formed by lateral com- pression through cooling. If we can make an estimate of these, and compare it with that which we have already made of the existing inequalities, we shall be able to form an opinion as to whether the cause assigned is, or is not, adequate for the purpose. Now it 1s fortunate that Sir William Thomson has already paved the way for such an investigation, by establishing the law which the temperature within the earth must follow, if it be a solid cooling by conduction. We are consequently able to F, 18 GONG oOo SS (CO)