Physics of the earth's crust

📙 This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1881 Excerpt: ...lava; the horizontal extent of that projection corresponding rudely with the horizontal extent of the table-land, and the depth thus gained is roughly equal to the increase of weight above from the prominence of the tableland2." 1 Ibid. p. 102. 2 Ibid. p. 103. It will be at once seen that this reasoning produces an arrangement of the crust upon the subjacent fluid exactly analogous to that which has been arrived at in the preceding chapter. Of course we from our point of view cannot contemplate the independent formation of a table-land apart from the downward projection, so that it should afterwards break and form one. But it does not seem that it concerned Sir G. B. Airy to show that the one cannot be formed without the other. The object he had in view was to explain how this arrangement of matter would produce a smaller deviation of the plumbline. He says, "It will be remarked that the disturbance depends on two actions; the positive attraction produced by the elevated table-land, and the diminution of attraction produced by the substitution of a certain volume of light crust (in the lower projection) for heavy lava. The diminution of attractive matter below, produced by the substitution of light crust for heavy lava, will be sensibly equal to the increase of attractive matter above. The difference of the negative attraction of one and the positive attraction of the other, as estimated in the direction of a line perpendicular to that joining the centres of attraction of the two masses (or as estimated in a horizontal line), will be proportional to the difference of the inverse cubes of the distances of the attracted point from the two masses." "Suppose then that the point C is at a great distance, where nevertheless the positive attrac...