WEIGHT AND SUPPORT IN SHIPS. 
457 
horizontal fluid pressure in a ship at sea. We have already seen that this pressure has 
an effect too considerable to be neglected in certain classes of ships floating in still water ; 
and that in a ship like the ‘ Belleroplion’ the effect might rise to one fourth of the 
bending-moment due to vertical forces. I have also stated that in ships at sea the con- 
ditions of strain are very different, and that no great error would be introduced by 
neglecting the effect of the horizontal fluid pressure, and considering only the vertical 
forces. This I shall now proceed to show, availing myself once more of the two ex- 
treme positions of support illustrated by figs. 14 & 15 (p. 446). 
First, let us consider the case of a ship on a wave-crest. We have already seen that 
the vertical forces incidental to that position had the effect of making the maximum 
bending-moment from three to four times as great as it was in still water ; but it will 
be evident that the effect of the horizontal fluid pressure does not increase in that ratio, 
if it increases at all. A glance at fig. 14 shows that the depression of the water-level at 
the bow and stern must have the effect of uncovering, so to speak, a large portion of the 
midship section at the middle, and so greatly reducing the pressure on that midship 
section below what it would be if the water-level at that section were preserved through- 
out the length*. In addition to this, at a wave-crest the tension and pressure of the 
fluid are reduced by means of the vertical motion ; so that both these causes, acting con- 
jointly, produce such a reduction of the bending effect of the horizontal pressure as to 
render it, in all probability, no greater than, even if so great as, that of the correspond- 
ing pressure in still water. Consequently in proportion to the bending-moment due to 
the vertical forces, that produced by the horizontal pressure may be safely neglected for 
this position. 
The same thing is true for the wave-hollow, only for a different reason. In that posi- 
tion, as shown by fig. 15 (p. 446), the water-level at the extremities of the ship is higher, 
and that in the middle lower, than in still water. Hence it is obvious that, for some 
portion of her depth lying above the centre of gravity of the midship section, the ship 
will be subjected to compressive strains, as well as for that portion lying below the 
centre of gravity. The effect of this will evidently be to reduce the bending-moment 
of the horizontal pressure ; and this reduction will in all probability more than counter- 
balance the effect of the increased tension of the water in the wave-hollow. On this 
account we may assume that the effect of the horizontal pressure may be neglected in 
comparison with that of the vertical forces, the latter being, as we have seen, from three 
to six times as great in the wave-hollow as it is in still water. 
These general considerations are confirmed by roughly approximate calculations made 
for the ‘ Bellerophon ’ when on the crest and in the hollow of waves of her own length. 
From what has just been said, it will appear that in making such calculations we shall 
obtain results considerably exceeding the true ones if we assume that the pressure is 
always due to the maximum height of the water-level on the ship ; so that if these 
* It is true that an additional area of midship section is covered at the sides by the rise of the wave-crest, 
but, as the nest sentence mentions, the tension and pressure of this wave-crest are small. 
