RESISTANCE OF IRON PLATES TO PROJECTILES. 283 



and the ultimate elongation of iron indicates its resistance; or, in other words, 

 the product of the tenacity and ductility of iron affords a measure of the 

 dynamic resistance of the material, or its resistance to impact. The following 

 numbers give this coefficient of rupture : — 



Mallet's coefficient in foot pounds. 

 Thinner plates. Thicker plates. 



A plates 1941 7544. 



B plates 1716 6476 



C plates 6593 8265 



D plates 493 5115 



To ascertain this coefficient with accuracy, rather longer specimens should 

 have been tested ; but, bearing in mind this source of inaccuracy, the numbers 

 strikingly correspond with the results obtained by impact. It is not of much 

 use to compare directly the resistances obtained with those givenabove, because 

 the former were made with such large intervals (half- inch) in the thickness 

 of the plates that they afford no criterion of the relative values of the dif- 

 ferent descriptions of iron. But we may compare the iron and steel plates, 

 where the difference of resistance, being greater, is to some extent indicated 

 in the experiments with ordnance. 



Dynamic resistance. 

 Thickness of plates. Iron plates. Steel plates. 



Half inch 1-00 1-72 



One and a half inch I'OO 1*19 



Two inches 1-00 1-20 



Two and a half inches 1*00 1'17 



Three inches I'OO 0-88 



With these results obtained by simple pressure, we compare those obtained 

 by ordnance. The resistance of the iron plates being again taken as unity, 

 the resistance of the steel plates was as follows: — 



Weight of Mean thickness Dynamic resistance, 



projectile. of plates. Iron. Steel. 



0-344 -75 1-00 1-97 



6-25 1-20 1-00 1-18 



11-56 1-75 1-00 1-00 



24-81 2-38 1-00 0-81 



From the above it will be seen that there is quite as close an approximation 

 in the ratios in these two tables, for corresponding thicknesses of plate, as 

 could be expected from the nature of the experiments. Both the series of 

 experiments (viz., that with dead pressure and that with ordnance) indicate 

 the same increasing resistance of the iron plates, and decreasing resistance of 

 the steel plates ; and the ratios of their relative resistances are nearly the 

 same. In making the comparison, the resistance to ordnance is assumed 

 to be as the square of the thickness of the plates — a law which will here- 

 after be demonstrated. 



The relative values of the plates in resisting shot are, according to the ex- 

 periments with dead pressure, as follows : — 



A plates 1000 



B plates 858 



C plates 1095 



D plates 688 



These numbers are deduced from the results on the I5, 2, 2|, and 3-inch 

 plates. With 3-inch plates the iron is much stronger than the steel. 



