Penny Blood: Serial Sensation and Working-Class Entertainment — page 143: what you’re looking at

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

126 DISTURBED TRACT. [cH. x. been from 17 to 21 miles. But since it is very improbable that any portion of the neutral zone should be brought so near the surface as to be exposed, we may conclude that the crust is thicker than when thus estimated. Again, Mr Mallet has determined the temperature of melting slag to be about 3000°F.’, and we can hardly suppose that the materials of the crust could be solid at such a temperature, as can melt silicates without the presence of water. This tempe- rature would, at the same rates of increase, be found at the depths of about 28 or 30 miles, consequently we may conclude from this argument that the crust 1s certainly less than 30 miles thick, One of these reasons then leads us to think that it is more than 17 or 21, and the other that it is less than 28 or 30 miles in thickness, according as we assume the higher or lower rate: and we have given reasons for preferring the higher*®, Per- haps we shall not be very wrong in assuming 25 miles as its average thickness between the upper and lower mean levels. In equation (1) » is the whole force which resists motion upwards, and at the neutral zone it 1s equal to h gph+ | f da; which consists of two parts, namely the weight of the column of rock above that zone and the frictional force above it. Similarly p’ in equation (2), taken between the neutral zone and the bottom of the crust, gives the whole force which resists motion downwards, and this is found to be - rk gpk + | , fdx. The first term of this expression is the same as in the previous one, and the second term is the frictional force below the neutral zone. By the property of the neutral zone these second terms are equal, and will evidently increase with the thickness of the crust. So also will the first term. And, since the’ depth A of the neutral zone has been shown to be proportional to the thickness of the crust, it follows that the first term is propor- tional to that thickness, and the second possibly may be s0. 1 p, 68, 2 p. 16. ECOMICOOOKS.COR % —SEap an =a