144 THE LONGITUDINAL STRENGTH OF RIGID AIRSHIPS. 
form together with an inner longitudinal, the N-girder, a triangular “keel girder.” The polyg- 
onal form of the structure is preserved by means of main frames spaced 10 meters apart, 
assisted by intermediate frames placed midway between the main frames. The longitudinals 
and the frames are all connected with each other at their points of intersection, but the con- 
nection between the secondary longitudinals and the main frames is very indirect. The points 
of intersection are referred to in the following as the joints. The structure so far described 
is incapable of resisting shearing and torsion. If there were no interconnections between the 
longitudinals, they would be able to move lengthwise relative to each other without meeting 
much resistance; the frames could distort freely, and the whole ship could twist around its 
axis. In order to prevent these actions the rectangular panels formed by the main longitu- 
dinals and the main frames are each of them spanned by two diagonal wires, which we shall 
refer to as the “shear wire” and the “counterwire,’ meaning by the former that wire which 
takes the shearing load in any given case. When an airship is in the hogging condition, the 
wires that are inclined downwards towards the bow forward of the section of maximum bend- 
ing moment and downwards towards the stern aft of that section are the shear wires. 
Another set of somewhat lighter wires span in the same way the panels which are bounded 
horizontally by the main longitudinals and vertically by consecutive main and intermediate 
frames. These wires, shown dotted in Plate 53, are referred to as the secondary wires; they 
are omitted in certain parts of the bottom, but are fitted completely in the upper part of 
the ship and in those parts of the bottom where the cars are suspended. Locally there are 
deviations from this system of wiring, but on the whole there is almost perfect continuity in 
form and structure, more so than in the shell construction of an ordinary ship. The main 
frames are stiffened in their own plane by a system of diametral and chord wires, which 
effectively prevent distortion in a transverse plane and play the same part in the structure as 
transverse bulkheads in an ordinary ship. 
The structure of an airship differs essentially from that of an ordinary ship in that the 
shearing is taken entirely by wires incapable of resisting compression, although not, as shown 
in the following, incapable of transmitting compressive forces so long as they are taut. When 
an ordinary ship is subject to shearing, the lines of principal tensile and compressive stress 
run continuously along the shell plating, crossing each other at right angles. When subject 
to combined shearing and bending, the shearing stresses attain their maximum value at the 
neutral axis and the lines of direct stress are here inclined at 45 degrees to the horizon.* 
In an airship the lines of stress will tend to arrange themselves in the same way, but this is to 
some extent prevented by the peculiarities of the wired structure. While the shear wires 
are in tension, the counterwires, even if set up with an initial tension, are liable to be slack 
when the shearing deflections are very great. The tensile stresses at the neutral axis are 
able to act along the shear wires in diagonal lines, much as they do in the shell of an ordi- 
nary ship, but if the counterwires are slack, the principal compressive stresses, which tend 
to act at right angles to the tensile, find no direct path and are forced to flow partly through 
the longitudinals, partly through the frames, which together form the rigid skeleton of the 
ship. At the joints, however, we may expect to find the stresses distributed as in a con- 
tinuous plated structure and the principal direct stresses at the neutral axis inclined at 45 
degrees to the horizon. The conditions are similar to those which would exist on the sides 
*The general flow of the stresses in the shell plating of a ship is illustrated in Fig. A, Plate I, page 46, in 
the author’s book on “Structural Design of Warships.” 
