363 
lEON SHIELDS FOE FOETS. 
By S. J. MACKIE, Assoc. Inst. C.E. 
T he subject of iron shields having been so very promi- 
nently brought before the public during the past few 
months, it may well and properly in its scientific aspect find a 
place in this popular journal. It has now come to be regarded 
as essential by the most advanced military countries to employ 
iron for the defence of the most vital and exposed parts of 
fortifications, to protect the costly and powerful guns with which 
such portions are or will be armed. "Without going into all the 
reasons which have rendered the use of iron imperative, we may 
briefly dwell upon the necessity of keeping out the terrible 
modern missiles from the works, the quick destruction effected 
by heavy rifled artillery upon granite and stone walls, and the 
increasing difficulty of dealing with earthworks of the enor- 
mous thickness now required in respect to the increased expo- 
sure of the guns and men in the embrasures. It will be seen 
at a glance that as the opening through which the gun is fired 
must be wide enough to allow for its training, even a masonry 
work of the minimum thickness must, by being cut away on the 
1 diagonal, be considerably weakened at the very place where the 
greatest amount of strength would be required. 
, What becomes a serious defect in a seven feet thick wall, 
I becomes frightful in mounds of from six to nine times that depth. 
! In both cases, therefore, the necessary opening for the use of 
j! the gun would be properly closed by an iron shield ; the metal 
I admitting at once of both a stronger as well as a thinner 
j defence. An' iron wall all round the fort is not essential, nor 
I even desirable, except where a number of guns have to be 
; crowded together within a very restricted area. If not broken 
1 1 into by openings, the simple earthwork would be preferable for 
I the defence for many other reasons than the economic one 
i j of cost. 
•I The problem then to be solved in the construction of the 
' iron shield is not a light one. We have to provide in it such 
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