258 ON THE DEFENCES OF POET JACKSON, 



penetrated 5 feet in a 9 feet brickwork before bursting. [The 

 Shunt gun caused more damage than the Whitworth, on account 

 of the larger area of the shot and the greater charge of powder 

 contained in the shell.] 



The effect of Artillery against good rubble masonry was 

 tested as far back as 1834 at Metz, in France, and it was found 

 that at 1094 yards, a 36 Ibs. projectile fired with 12 Ibs. of 

 powder produced a mean penetration of 1 8.2 inches. 



As heavy rifled artillery has a proportionately greater effect 

 than the old smooth bore guns, it would be unsafe to trust to 

 masonry for our batteries, and at least that portion of our works 

 likely to be exposed to the fire of heavy rifled ordnance ought to 

 be covered with armour plates, and the best section of iron wall 

 for us to adopt must be determined by comparing the experiments 

 on the different iron targets. 



As a notice of the relative merits of the various targets tested in 

 England and other countries would alone suffice to fill a very long 

 paper, I shall only mention a few experiments shewing the results 

 of the trials of the best targets in connexion with the ordnance 

 used against them. I have illustrated many of the target experi- 

 ments by drawings on which I have also compiled tables shewing 

 the effect produced upon them by the several kinds of ordnance. 



The construction I have suggested for our exposed forts and 

 batteries is calculated to offer the greatest amount of resistance 

 to heavy shot with the least possible thickness of iron wall. With 

 the rigidity of the backing and framing, I combine the resistance 

 of solid plates 6 in. thick, and provide the bolts with a peculiar 

 compression washer. The external openings of my embrasures, 

 are hardly larger than the internal openings, and are to be fitted 

 with folding shutters of 6| in. forged plates. 



I provide for any re-inforcements that may be required at 

 any position by making my batteries large enough to have a heavy 

 gun placed in each embrasure if necessary. The expense of 

 plating the exposed sides of a large circular or oval Fort is not 

 very considerable, compared with the advantages to be derived 

 from being able to use additional guns safely in the batteries. 

 The difficulty of procuring heavy rifled guns now, and their great 

 cost render such provision necessary ; and if good roads are made 

 between the different works, the heaviest guns might be moved 



