the tendency is for the centre of the ship to bend upwards and the unsupported ends to droop. This is technically called “hogging.” In the reverse circumstance, when the ends are supported on the tops of two mountains of waves, whilst the centre of the ship spans, unsupported, the intervening valley, the tendency is to “sag.” Now this has to be allowed for in the construction of the ship, and, as already pointed out in my “Sailing Ships and Their Story,” this was understood as far back as the times of the Egyptians, who counteracted such strains as these by means of a longitudinal cable stretched tightly from one end of the ship to the other. But with the coming of steamships there was another problem to be taken into consideration. Engines, boilers, fresh water for the boilers, coal and so on are serious weights to be placed in one part of the ship. (In the case of the _Great Western_, the first three alone weighed 480 tons, although the gross tonnage of the whole ship was only 1,321.) Throughout the length of the ship, then, she is subjected not merely to irregular strains by the peaks and valleys of the waves, but by the distribution of weights. Her structure has to undergo the severest possible stresses, and these are different when the ship is loaded and when she is “light.” If you divide a ship into sections transversely, as is actually done by the designer, you will find that some parts are less buoyant than others, no matter whether your ship is made of wood, iron, or steel. Those sections, for instance, which contain a steamer’s machinery will have much inferior buoyancy, and, indeed, were you to sever them from the ship and seal them up so as to be perfectly water-tight, they would in many cases sink. Therefore, this irregularity of buoyancy has to be met by making the more-buoyant sections help to support the less-buoyant. In actual shipbuilding practice it is customary to regard the greatest stress to a ship as occurring when she is poised on the crest of a wave, and it is usual to suppose, in order to safeguard her manner of construction, that she is poised upon the crest of a wave whose length from trough to trough is equal to the length of the ship, and the height of the wave from trough to crest to be one-twentieth of its length when 300 feet long and below, and one twenty-fifth when exceeding that length.