The block, for our present purposes, may be regarded as _monolithic_, either being really so or as a mass composed of parts so united by joggles, treenails and mortar, as to be free from any tendency to disintegration by the force of the waves; and in this case the stability of the cylinder will depend upon the amount of friction opposing the pressure of the waves which tends to produce a sliding movement. It appears, by some experiments of M. Rondelet,[10] that the friction of a block of stone sliding on a chiselled floor of rock is equal to ⁷⁄₁₀ths of its own weight; and we should thus obtain in the present instance 168 tons, as the amount of friction tending to resist the pressure of the waves, which would therefore exert a power superior to that resistance by 74 tons.[11] But this excess of force would be easily neutralized by the adhesion of the mortar and the abutment of the block against the sides of the foundation pit into which Lighthouse Towers in such exposed places are generally sunk in the solid rock. When, in addition to these considerations, we learn that the solid frustum, or lower part of the Eddystone Tower, which has weathered so many storms for the last ninety years, does not greatly exceed in mass the imaginary cylindric block which I have spoken of, our confidence in the stability of the cylinder is greatly increased. Our belief receives a still farther confirmation from the fact, that the strongest instance recorded of the power of the waves, falls considerably short of the case which we have just imagined. The instance alluded to is given in Mr Lyell’s Geology, on the authority of the Reverend George Low, of Fetlar, in Zetland, who mentions, that a block, whose dimensions seem to give us reason to estimate its weight at nearly 300 tons (or about _one-fifth_ less than that of the cylinder), was moved over a point, and thrown into the sea; and it must be remembered, that the form of this block, which was only 5 feet thick and about 40 feet long, rendered it very susceptible of a sliding motion, and must have greatly aided its transport. We may therefore not unreasonably conclude, that, in designing such a tower, it is safe to assume a mass which our own judgment and recorded facts seem to concur in pronouncing beyond the power of the greatest waves, as fixing the _lowest_ limit to which the contents of the proposed edifice may be reduced. [8] This reduction seems to be warranted by the results of some experiments of Bossut. [9] This is the product of 240 tons, by the ratio of 2·5. [10] L’art de bâtir. [11] The number 168 is ⁷⁄₁₀ths of 240, which is the weight of the cylinder, reduced by the weight of an equal bulk of salt water; and 74 is the excess of 242 tons, the pressure of the waves, above 168, the amount of friction. * * * * *