Penny Blood: Serial Sensation and Working-Class Entertainment — page 173: what you’re looking at

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

156 THE REVELATIONS OF (cH. XII. Hence we must have dv , a, B=, and .° dv 4, “dn S ° This gives the proportion according to which the average rate along any normal to an isogeotherm is diminished in con- sequence of the convexity of its surface. This rate will clearly become more and more nearly equal to that through the plane isogeotherm, as that 1s approached. Now the whole length of the St Gothard tunnel is 14920 metres, and the greatest height of the mountain above it is 1127 metres, It is therefore obvious that the ratio f must be nearly one of equality. Indeed, if we suppose the contour of the surface of the mountain to be a segment of a circular cylinder, we shall find that the value of 7 for the outer surface, where that ratio will differ most from equality, will be 0°996. And consequently, if we suppose that the temperature of this sur- face is everywhere 0° C. (an unfavourable supposition), then dv , This shows that the convexity of the mountain can only slightly affect the rate along the vertical anywhere, and in the central part, where = becomes = , scarcely at all. We are therefore d at liberty to regard the rate in the central part of the mountain as very nearly constant and equal to 7’. Now the temperature at the upper surface being 0°C., if h be the height of the mountain above the tunnel, and ¢ the temperature of the rock in the centre of the tunnel, the rate pa i But at the depth where the plane isogeotherm 1s encountered the temperature beneath the mountain must equal that of the COMIC OOOKS.EO