256 report — 1865. 



It may not be practicable in all cases to construct iron ships with such a 

 large section of iron at the upper deck ; yet such should be the distribution of 

 the material in the section in order to secure a maximum of strength with a 

 given amount of material. 



In the distribution of the material there is another consideration of some 

 importance ; and that is, that all bodies in the form of beams, whether hollow 

 or solid, follow the same law as regards a transverse strain, viz. that in a beam 

 uniformly loaded the strains are always greatest in the middle and progres- 

 sively diminish to the points of support at either end. These facts arc self- 

 evident, and show in the case of an iron ship that the same thickness of plates is 

 not required when working from the centre at midships to the stem and 

 stern. In fact, they should taper or be reduced in thickness according to 

 a certain ratio of their distances from the centre till they reach the extremes 

 at each end. Theoretically this is true ; but in practice we have to consider 

 how much the thickness can be reduced without danger to the structure, and 

 in general we may here observe that the reduction should not exceed one-third 

 between the centres and the two extremes. Or, in other words, if we assume 

 the strakes or sheathing plates of the bottom and round the bilge to the height 

 of the interior floor, or one-fifth of the depth, to be seven-eighths of an inch, 

 it then follows that their thicknesses may be safely and progressively reduced 

 to five-eighths thick towards the bow and stem. The same reduction to 

 five-eighths may be made from that point, one-fifth of the depth, to the 

 neutral axis of transverse strains, or about halfway up the ship's side, when 

 they shoidd again increase to seven-eighths thick for the top strakes at the 

 deck, on each side, where they have to perform the office of stringers and 

 columns under the action of the two forces of tension and compression. 



From these remarks it is obvious that a careful distribution of the material 

 is a consideration of great importance in ship-building ; and although it may 

 be necessary in some constructions to deviate somewhat from the absolute 

 rule, yet it is nevertheless essential that the law of strains should be carefully 

 observed, and weak parts sufficiently guarded against. 



Section V. 

 On the Penetration of Iron Armour-Plates by flat-faced tempered Steel Shot. 



25. The experiments conducted at Shoeburyness seem to warrant the con- 

 clusion that, in firing at strong armour-plates with cast or wrought-iron shot, 

 about one-half of the effect is lost in distorting or in breaking up the shot 

 itself, and, further, that with steel shot of tough temper nearly the whole 

 of the work stored in it is expended upon the plate. 



Mr. Fairbairn's experiments on punching led to the following general laws 

 relative to the resistance of iron plates to a force tending to rupture them : — 



The diameter of the punch being constant, the pressure requisite to pro- 

 duce rupture varies, approximately, as the thickness of the plate. 



"When the thickness of the plate is constant, the pressure requisite to pro- 

 duce rupture varies, approximately, as the diameter of the punch. 



And, generally, the pressure requisite to produce rupture varies as the 

 thickness of the plate multiplied by the diameter of the punch, or, what 

 amounts to the same thing, as the area of the shearing surface abraded in 

 the process of punching ; that is, 



Vttrt, 



where P is the ultimate pressure in lbs., (the thickness of the plate in inches, 

 and r the radius of the end of the punch. 



