WEIGHT AND SUPPORT IN SHIPS. 
455 
middle of a ship’s length, and the after body is lying across the hollow, we should 
expect to find hogging-strains forward and sagging-strains aft; and as the wave-crest 
moved aft relatively to the ship, the character and intensity of the bending-strains must 
be continually varying in passing from one extreme to the other. Of the extent to 
which this variation reaches we already have some idea ; but we must also introduce the 
idea of its rapidity before we can realize any thing like its full effect. 
In order to make my remarks as definite as possible, I will again refer to the three 
typical ships on waves of the dimensions previously assigned. The time of transit of 
waves 400 feet long is rather less than nine seconds (really 8*83 seconds), and in half that 
time, or 4| seconds, we may suppose the ‘ Minotaur’ to have passed from the wave-crest 
to the wave-hollow; while her maximum bending-strain has changed from 140,000 foot- 
tons of hogging- to 74,000 foot-tons of sagging-moment; and every transverse section 
has been subjected to similar changes in the character and amount of the strains brought 
upon it. The great rapidity in the variation of the strain is so apparent from this brief 
statement of facts that I need not dwell upon it. 
The ‘"Victoria and Albert’ and the ‘ Bellerophon’ have been supposed to float on 
waves 300 feet long, for which the whole time of transit is less than 8 seconds (really 
7*65 seconds). In half that time, therefore, or in less than 4 seconds, the ship may have 
passed from crest to hollow, and the bending-strains have passed through successive 
phases from one extreme to the other. On the crest the maximum hogging-moment 
of the ‘Victoria and Albert’ has been found to be about 16,400 foot-tons; in less than 
4 seconds, in the wave-hollow, the maximum sagging-moment may have reached 31,000 
foot-tons, at the same section of the ship which previously had to resist the hogging- 
strain. This is, proportionately, the greatest change which we have met with. In the 
‘ Bellerophon’ the change in the same brief interval, although not so great in proportion, 
is very striking; for on the crest the hogging-moment amidships is 43,600 foot-tons, 
and in the wave-hollow the sagging-moment is 48,800 foot-tons. 
I give these figures merely as indications of what may be expected to happen in the 
changes of strain in ships at sea ; and they probably fall much below the truth, since, as 
I have just said, no account has been taken of the effect of violent pitching-motions, 
which must lead to still more abrupt and violent changes. Enough has been said, 
however, to show how important this feature of the subject is ; and I will simply add 
that a very convenient way of expressing the effect I have been attempting to describe 
is afforded by the supposition that the ship is fixed, and that what may be termed 
“waves of strain” roll through her structure. The introduction of this idea will help 
us to understand more clearly how changes in strain affect a structure ; for a very small 
strain (considered statically), which would not affect a comparatively weak structure 
sensibly if it were constantly acting in one direction, will suffice to destroy a far stronger 
structure if its direction is continually and rapidly changed. 
This subject has not escaped the attention of preceding writers ; and Mr. Faiebaien 
has made some interesting remarks upon it, at page 13 of his work on ‘Iron Ship- 
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