possible to effect a landing, it has been thought necessary that there should never be fewer than three Keepers on duty. This addition to the ordinary establishment of a Lighthouse calls for a greater number of sleeping-cabins, and, at the same time, involves a corresponding increase in the supply of water, fuel and other provisions, requiring much additional stowage. So far, therefore, a Light Tower in an exposed situation, differs from one on shore only in the extent of its internal accommodation. The second class of considerations, which must guide the Engineer in framing a design for a Light Tower which is exposed to the force of the waves, refers solely to the stability of the building. The first observation which must occur to any one who considers the subject is, that we know little of the nature, amount and modifications of the forces, on the proper investigation of which the application of the principle which regulates the construction must be based. When it is recollected, that, so far from possessing any accurate information regarding the momentum of the waves, we have little more than conjecture to guide us, it will be obvious, that we are not in a situation to estimate the power or intensity of those shocks to which Sea Towers are subject; and much less can we pretend to deal with the variations of these forces which shoals and obstructing rocks produce, or to determine the power of the waves as destructive agents. No systematic or intelligible attempt has been made practically to measure the force of the waves, so as to furnish the Engineer with a _constant_ to guide him in his attempts to oppose the inroads of the ocean. The only experiments, indeed, on the subject, with which I am acquainted, are those of Mr Thomas Stevenson, Civil-Engineer, who had long entertained the idea of registering the force of the impulse of the waves, and lately contrived an instrument for the purpose, which he has applied at various parts of the coast. I therefore gladly avail myself of the present opportunity, to give a brief statement of the results indicated by it, as contained in a paper by the inventor, which appeared in the Transactions of the Royal Society of Edinburgh of 20th January 1845, and of which a digest will be found in the Appendix, as any attempt to throw light upon this most obscure, but highly important subject, cannot fail to be interesting, not merely to the philosopher, but to the Marine Architect. It would naturally be expected, that the force of the waves should vary according to the season of the year, and the nature of the exposure, and this expectation is fully justified by the indications of the Marine Dynamometer. Thus it appears, that during five summer months of 1843 and 1844, the average indications registered at different places near Tyree and Skerryvore, gave 611 lb. of pressure per square foot of surface exposed to the waves; while the average for the winter months for the same places