THE ROYAL ARTILLERY INSTITUTION. 
433 
“ Whatever may be selected as the material of the interior structure of the fort, 
the exterior must be iron. Lines of earthworks will also be useful in assisting and 
completing the position of the main work, but by no means should bare masonry be 
exposed to the action of rifle cannon.” 
But as we are never likely to have iron forts, experiments have been made 
with a view of obtaining the best description of shield for the protection of 
the guns and detachments; these shields will also strengthen the parapets 
at their weakest points. 
The accompanying sketch shows a section of Thornycroft bars. 
As far back as 1859 
an iron embrasure com¬ 
posed of bars of this 
description 14" thick 
and 4" deep, was tested 
at Portsmouth by the 
Ordnance Select Com¬ 
mittee, at a range of 
400 yds.; the bars were 
fixed together by bolts 
passingverticallythrough 
them. The result of the 
experiment was reported 
as affording “very good 
ground for believing that 
iron screens or targets 
made on this plan (with 
certain improvements in 
their details of construc¬ 
tion) will resist the 
heaviest shot and will 
prove of the greatest 
value in protecting casemates.” A second trial of this system was made in 
1860 on bars 10" thick, but the bad method of fastening employed in the 
previous shield by means of vertical through bolts, was abandoned, and the 
bars were held together at the back by 3" vertical tie bolts at each end—the 
shield was tested, both with, and without, a masonry backing, at 600 and 
400 yds. ranges, and received 21 direct blows when backed, and 29 after the 
stone backing had been removed and when an abutment of 30" only of stone 
had been left at each end of the shield;—the Committee expressed them¬ 
selves satisfied with this mode of construction as thoroughly efficient for 
the purpose in view, and stated that the stone backing might safely be 
dispensed with, as it appeared to add little, if anything, to the power of 
resistance of the shield. 
The heaviest shot, however, used in these experiments was 97 lbs. weight, 
fired from an Armstrong 80-pr. rifled gun with a 12 lbs. charge. 
In consequence of the favourable nature of these reports, the Special 
Committee on Iron caused two shields to be constructed on this plan in 
1861—one 10" thick, the other 8" thick—the heaviest gun used in the 
