But the magnitude of the strain on the different parts of a ship is subjected to very material alterations, when she is exposed to the force of the wind and waves. The effect of the wind is generally compen sated by a change of the situation of the actual water line, so that its amount may he estimated from the temporary or permanent inclination of the ship; and the force of the waves may be more directly calculat ed from their height and breadth. These two forces can seldom be so applied, as to combine their effects, in producing a strain of the same kind in their full extent; it will therefore be sufficient for this purpose, to determine the probable amount of the force of the waves, which is more materially concerned in affect ing the longitudinal curvature than that of the wind. As a fair specimen of the greatest strain that is like ly to arise from this cause in any common circum stances, we may consider the case of a series of waves twenty feet in height, and seventy in breadth; the form being such, that the curvature of the surface may be nearly proportional to the elevation or depression. A single wave might indeed act more powerfully than a continued series, but such a wave can scarcely ever occur singly. Dr. Young then finds," that the great est strain takes place, in a seventy-four gun ship, at the distance of about eighteen feet from the midships, amounting to about 10,000 tons, at the instant when the ship is in a horizontal position, while, in common cases, when the waves are narrower, the strain will be proportionally smaller and nearer to the extremity. Hence it appears that the strain produced by the ac tion of the waves, may very considerably exceed in magnitude, the more permanent forces derived from the ordinary distribution of the weight and pressure; being according to this statement, nearly three times as great: so that when both strains co-operate, their sum may be equivalent to about 15,000 tons, acting on a lever of one foot, and their difference, in oppo site circumstances, to about 5000. There may pos sibly be cases in which the pressure of the waves produces a still greater effect than this; it may also be observed, that the agitation accompanying it, tends to make the fastenings give way much more readily than they would do, if an equal force were applied less abruptly. At the same time,it is not probable that this strain ever becomes so great, as to make the former perfectly inconsiderable in comparison with it, especially if we take into account the uninterrupt ed continuance of its action; it appears, therefore, to he highly proper, that the provision made for coun teracting the causes of arching, should be greater than for obviating the strain in the contrary direction; for example, that if the pieces of timber intended for opposing them were, on account of the nature of their fastenings, or for any other reason, more capable of resisting compression than extension, they should be so placed as to act as shores rather than as tics; al though it by no means follows, from the form which the ship assumes after once breaking, that the injury has been occasioned in the first instance, by the im mediate causes of arching; since, when the fastenings have been loosened by a force of any kind, the ship will naturally give way to the more permanent pres sure, which continues to act on her in the state of weakness thus superinduced.
The pressure of the water against the sides of a ship, has also a tendency, remarks Dr. Young, to pro duce a curvature in a transverse direction, which is greatly increased by the distribution of the weights, the parts near the sides being the heaviest, while the greatest vertical pressure of the water is near the keel. This pressure is often transmitted by the stanchions to the beams, so that they are forced upwards in the middle: when they are unsupported, the beams are more generally depressed in the middle by the weight of the load which they sustain, while the inequality of the pressure of the water co-operates with other causes in promoting the separation of the sides of the ship front the beams of the upper decks. On the other hand the weight of the mainmast often prevails partially over that of the sides, so that the keel is forced rather downwards than upwards in the imme diate neighbourhood of the mid-ships. The tenden cy to a transverse curvature is observable, when a ship rests on her side, in the opening of the joints of the planks aloft, and in their becoming tighter be low; although this effect depends less immediately on the absolute extension and compression of the neighbouring parts, than on the alteration of the cur vature of the timbers in consequence of the pressure.
In such a case also there is an obvious strain tend ing to produce a lateral curvature, and shores are some times employed• to prevent its effects, when a ship is "hove down" on her side. This indeed is compara tively a rare occurrence; but when a series of large waves strikes a ship obliquely, they must often act in a similar manner with immense force: the elevation on one side may be precisely opposite to the depression on the other; and the strain from this cause can scarcely be less than the vertical strain already calcu lated: but its effects are less commonly observed, be cause we have not the same means of ascertaining the weakness which results from it, by the operation of a permanent cause. AVhen a ship possesses a certain degree of flexibility, she may in some measure elude the violence of this force, by giving way a little for the short interval occupied by the passage of the wave; but it may be suspected that her sailing in a rough sea, must be impaired by such a temporary change of form.
Such arc a few of the general principles connected with that alteration of form, which has been denomi nated arching or hogging, and for which the genius of Seppings contrived so admirable a remedy. The symptoms of weakness which all ships constructed on the old plan exhibited, were generally apparent, by the parting of some of the butts of the plank aloft, at the same time that the angular position of sonic parts of the structure had as uniformly been more or less al tered; and very generally a certain degree of sliding was observable in the planks at the side of some of the ports. At the same time, a degree of permanent compression or crippling below was remarked, the butts of the planks opening when the cause that pro duced the arching had been removed, and the sheath ing more wrinkled than would have happened from the simple bending of the planks. But as the practical details connected with the introduction of the diago nal riders, belong more particularly to the part of the paper devoted to practical construction, we refer the reader to it for further information.