These general principles are much more readily and safely applicable to ships while floating in ‘still water’ than to ships when at sea—the strains experienced then being necessarily the results of far more complex and severe influences. The existence of waves and their rapid motions relatively to that of the vessel, and the pitching, heaving, and other movements thus caused, increase the inequality of distribution of weight and buoyancy and affect more materially the strains brought upon vessels. Consideration of the problem, therefore, involves a study of waves, both as to their formation and action, and necessarily leads to a mode of treatment which cannot have accurate regard for particular cases. Variable influences of immense importance are also constituted by the state of loading in vessels for merchant service. For a uniform basis of comparison in these calculations such vessels are usually assumed as loaded with homogeneous cargo—_i.e._, cargoes of equal density throughout. This fundamental element of relative ‘weight’ and ‘buoyancy’ having been indicated, the chief strains to which a ship is subjected may now be stated. This may be done with sufficient regard to general accuracy, under four heads:—[7] (1) Strains tending to produce longitudinal bending—“hogging” or “sagging”—in the structure considered as a whole. (2) Strains tending to alter the transverse form of a ship, _i.e._, to change the form of athwartship sections. (3) Strains incidental to propulsion by steam or sails. (4) Strains affecting particular parts of a ship, or “local strains”—tending to produce local damage or change of form independently of changes in the structure considered as a whole. To these might be added various other strains, which, however, are of less practical importance, and are not felt in any great degree—except in very special cases and under unusual circumstances—apart from the strains which affect the structure considered as a whole. The provisions made for the latter are, under ordinary circumstances, sufficient to cover the demands of the former, but particular cases may have to be provided for on their merits, apart from the treatment generally applicable. The manner of ascertaining the strength of a ship to resist strains tending to produce longitudinal bending, is to compute the effective sectional area of all the longitudinal items in the structure which are brought under compressive or tensile strain, and from this to calculate the strength in the same manner as for a girder having an aggregate sectional area and a disposition of material equivalent to that of the ship.