742 
= 360 <3 
the drop line securely since the fastenings on these mine cases failed easily. 
These cases were modified so that the strain from the drop line was assumed 
by the top of the mine case rather than by a weld at the bottom (Fig. 28A). 
Another advantage of this modification was that it allowed the insertion of 
a shock absorber to take up the shock occurring when the shock wave struck 
the mine case. In attaching the drop lines to the stcel oil drums it was 
found best to make two bands of iron which completely circled the drum at 
two points, running a bridle between these two bands and attaching the line 
to this bridle. Much difficulty was experienced with having the drop line 
snap under the impact of the shdck wave. Steel cable 5/16 in. in diameter 
has a breaking strength of roughly 6,000 1b and the heaviest weight put up— 
on these cables was not more than 200 lb. However, the upward component on 
the float due to the pressure and the shock wave gave sufficient accelera- 
tion to break these cablese This was overcome by the use of some sort of 
shock absorber which would take up this instantaneous acceleration. Two 
modifications which have been used successfully are shown in Fig. 28. 
In addition to the stcel drop lines, manila safety lines were employed 
so that in case a drop line parted the gear was not lost. The manila lines 
were also used for lowering gear (snubbing around a cleat on the deck) and 
for retrieving gear since they could be brought in over a winch head. It 
was customary to attach these to independent fastenings both on the float 
and on the gear which was being suspended so that in case of failure of 
either fastening the gear was not lost. 
The backward drag of the surface float caused variations in the depth 
of the gauges from the actual measured length of the various drop lines. 
This was not serious for the type of measurement being made on the RELIANCE 
Since an arbitrary limit of variations in depth up to 3 ft was allowed. 
However, if depth must be maintained cxactly, modifications to the type of 
rig here described must be made, 
(b) Horizontal componentse — Horizontal distances were maintained by 
keeping the horizontal spacer cable under strain at all times. This strain 
was the result of the 18=in, sea anchor at the stern end of the gauge spacer 
cable reacting against the 1 to 1.5-knot velocity of the gear as it was 
towed through the water. Near the sea anchor was a weight suspcnded from a 
surface float at the depth which it was desired to maintain. From this 
weight a manila line ran to the sternmost gauge block and 5/16~in,. steel 
cable was used for spacer lines from this block forwerd to other blocks. 
From the foremost gauge block the steel spacer cable ran forward to a 200-lb 
lead weight suspended at the proper depth and from here the tow line ran 
back to the RELIANCE at the surface. The mass of the lead weight was suf- 
ficient to overcome the upward component of the tow line. The drag on the 
tow line at the vessel was estimated to be 00 to 500 lb. The general lay- 
out of the gear for a fore and aft shot was modified from time to time, and 
the four main modifications are showm in Fig, 29 and 3. 
No lowering of gauge deformations due to the "shadow" of one composite 
gauge block on a second further from the charge was found at the time Modi- 
fication C was adopted. However, to minimize the possibility of any such 
shadowing, the rear block was hung on the opposite side of the spacer cable 
from the front block with a distance of at least 10 ft between the two. 
