RESISTANCE OF IRON PLATES TO PROJECTILES. 285 



Third Series of Experiments. 



In this series a punch of the same diameter as the bullet of the wall-piece 

 was employed, but it was round-faced, like the cast-iron service shot. The 

 same die was employed, the object being to determine the difference of pene- 

 trating poAver of round- and flat-faced projectiles. In the following table the 

 resistances are given, and those of the flat-faced punch of the first series are 

 placed beside them, for comparison. 



Statical Resistance to punching in lbs. 

 Flat-faced punch. Round-faced punch. 



f A plates 57,956 61,886 



Half-inch I B plates 57,060 4.8,788 



plates. 1 C plates 71,035 85,524 



[ D plates 49,080 43,337 



Three-quarter- r B plates 84,587 98,420 



inch plates. \ D plates 82,381 98,571 



Means 67,017 72,754 



These figures show that the statical pressure required to punch plates of the 

 same thicknessis about the same, whether the punch be round- or flat-faced. 



It is further shown in a detailed manner that, for the same pressure, the 

 volume displaced by indentation is the same for flat- and round-faced punches. 



Thence it follows that, where the plate does not exceed in thickness the 

 diameter of the punch, the depth of indentation is much greater with round- 

 than with flat-faced punches. 



And lastly, since the dynamic resistance which corresponds with thei'esist- 

 ance to projectiles, varies as the product of the statical pressure and the depth 

 of indentation, it thence follows that the dynamic resistance to round-faced 

 projectiles is much greater than the resistance to flat-faced projectiles. 



The general laws indicated in these experiments are as follows : — 



1. Size of shot or punch. — The resistance varies directly as the circum- 

 ference of the shot. 



2. Statical resistance of plates of different thickness. — With plates of 

 different thickness the statical resistance varies directly as the thickness. If 

 the thicknesses be as 1, 2, 3, &c., the resistances will be as 1, 2, 3, &c. 



3. Indeyitation. — The ultimate indentation can only be approximately ob- 

 tained during experiments on punching; it varies directly as the thickness 

 of the plates. For flat-faced punches we may assume it to be one-half the 

 thickness, and for round-faced punches the whole thickness of the plate, 

 when the thickness of the plates is less than the diameter of the shot. 



4. Dynamic resistance, or resistance to projectiles. — The dynamic resistance 

 varies as the product of the statical resistance and the ultimate indentation 

 of the plates. But both these quantities vary nearly as the thickness of the 

 plates, directly. Hence the dynamic resistance varies in a ratio which is 

 nearly that of the squares of the thicknesses of the plates. So that if the 

 thicknesses be as 1, 2, 3, 4, &c., the dynamic resistances will be as 1, 4, 9, 16, 

 &c. And the dynamic resistances will be nearly twice as great for round- 

 as for flat-faced projectiles. 



7. Computation of a general Formula for the Resistance of Iron Plates to 



Projectiles. 



Assuming the laws stated above as the result of the experiments on punch- 

 ing, the following formula has been deduced by equating the dynamic re- 

 sistance to the work accumulated in the shot. 



