[7] It was chiefly on these grounds that the Commissioners of Northern Lights, after consulting a Committee of the Royal Society of Edinburgh, and Messrs Cubitt and Rennie, Civil Engineers, rejected the design of Captain Sir Samuel Brown, R. N., who volunteered a proposal to build an Iron Pillar at the time that the erection of the Skerryvore Lighthouse was determined on in 1835. * * * * * Having satisfied himself that _weight_ is the most eligible source of stability, the next step of the Engineer is to inquire what quantity of matter is necessary to produce stability, and what is the most advantageous form for its arrangement in a tower. The first question, which respects the mass to be employed, is, as already stated, one of the utmost difficulty, and can be solved by experience alone, directed by that natural sagacity which Smeaton, in his account of his own thoughts on the subject, with much _naïveté_, terms ‘_feelings_,’ in contradistinction to that more accurate process of deduction which he calls ‘_calculation_.’ It is very difficult, for example, to conceive that the waves could displace a cylindric block of granite, 25 feet in diameter and 10 feet high, which would contain about 380 tons, and we almost _feel_ that they could not do so. If, in order to test the soundness of this expectation, we appeal to such experience as we possess, and apply to the _largest vertical section_ of such a solid, the greatest force yet indicated by my brother’s Marine Dynamometer, which, as already stated, is 4335 lb. per square foot, we shall obtain a pressure of 484 tons, which, being reduced by _one-half_[8] for the loss of force occasioned by the convexity of the opposing cylindric surface, gives 242 tons, as the greatest force of the waves tending to displace the cylinder. But in the extreme case we have now supposed the solid will be entirely immersed in the water, and its efficient weight will thus be reduced by 140 tons, or the weight of an equal bulk of sea-water; and the remaining weight of 240 tons, by which it will resist the force of the waves, will be almost exactly equal to the pressure which they exert. This imaginary cylinder may, however, be regarded as still within the limits of safety, because the waves could not overturn it, unless their pressure exceeded the weight of the block in a ratio greater than that of its diameter to its height, which in this case is that of 25 to 10, or 2¹⁄₂ times. In order, therefore, to endanger the stability of the solid by overturning it, the pressure, instead of being 240 tons, must be 600 tons.[9] We have thus seen, that the cylinder is secure from the chance of being overturned; but we have yet to consider how far it is exempt from the risk of being displaced by the pressure of the waves, causing it to slide along the surface of the Rock, owing to deficiency of friction between the two surfaces in contact.