128 TRANSACTIONS OF THE RoyAL CANADIAN INSTITUTE  [VOL. XI. 
The submersible has a rather high centre of gravity and in submerged 
condition a low metacentre (centre of buoyancy), and hence small stiff- 
ness on account of the high position of the ballast tanks, while in the 
surface condition the stiffness is in some cases excessively great 
owing to the large area of the water-line. The submarine has a lower 
centre of gravity and, due to the low lying water tanks, it has a high 
metacentre in the submerged condition and small or moderate stiffness 
on the surface. The Laurenti type, where the ballast tanks are partly 
below the strength hull partly above or very high, are intermediate 
between the Germania and the Holland type in this respect. 
In order to obtain sufficient stability in the submerged condition 
submersibles must generally carry a considerable amount of keel-ballast. 
This of course is a drawback, but also most submarines carry some 
ballast. Part of the ballast is generally detachable, often referred to 
as a ‘‘safety keel’, to be let go in case of emergency. In passing from 
the light to the submerged condition and vice versa a point will exist 
where the stability is a minimum, being reduced by the presence of free 
water in the tanks. The designer must, therefore, carefully determine 
the conditions of stability in all intermediate positions in order to 
satisfy himself that a proper metacentric height is always maintained. 
If the stability vanishes at any point the boat may heel over to a con- 
siderable angle before equilibrium is restored or may even capsize. 
STRENGTH OF HULL. 
The hull of a submerged vessel is exposed to an external water 
pressure which is directly proportional to the depth of immersion. 
Already at a depth of 200 ft. the pressure is about 100 lbs. per sq. in., 
and since the depth of water in the ocean is generally more than 10,000 
{t., boats cannot be constructed to withstand the pressures at all depths 
which they may encounter. It is therefore necessary to assign a limit 
to the pressure head which a boat is required to resist. Generally 
boats will have no reason to go deeper than required to clear the bottom 
of vessels on the surface, that is to a depth of about 75 ft., but accident- 
ally they may descend involuntarily to greater depth. Usually the head 
to which submarine boats are tested is about 150 ft., although in some 
navies it is as much as 200 ft. A certain margin of safety is of course 
applied in the construction, but if the boat goes much beyond its test 
depth it is liable to collapse. 
In most boats the strength hull is of circular section as stated above. 
The Whitehead boats and other boats of the Holland type have 
internal frames. As seen from the midship section of the Danish boat 
Havmanden, the Whitehead boats in some cases have oval sections 
