at the ground, changing to Ay at a height z in the air; this

Pz
Ρ

is neglecting the variation of density of the air and gas in a height so small comparatively as y ft.

For the cylindrical gas bag on an air ship of diameter d ft, and p diameters long (Fig. 57), we may put

Thus for an air ship 500 ft long by 50 ft diameter, d = 50, p = 10, weighing 25 tons,

In the non-rigid air ship the cylindrical envelope forms one continuous gas bag, and this must be kept constantly inflated.

If this should become partially deflated, like an ordinary spherical balloon, the indentation would affect the stability as it runs from one end of the belly to the other; and in the middle it would cause the gas bag to lose longitudinal stiffness, and allow it to double up like a bolster, as in Fig. 58.

FIG. 58.

A ballonet is required, and this is an interior bag which can be inflated by an air-pump to fill up the space left vacant by the escape of the hydrogen through the appendix in the ascent, and to keep the balloon distended during the descent, and on the ground, in spite of gas leakage.

One ballonet at each end inside is useful, shown by the dotted lines in Fig. 57, to serve like a trimming tank of a steamer, for keeping the air ship on an even keel.

In the air ship of rigid type the hydrogen is carried in a number of separate gas bags enclosed in a lattice frame cage, and covered with a smooth envelope to diminish frictional drag, so that this type is equivalent to a series of spherical balloons harnessed together in a horizontal line.

The gondolas are suspended below, as in Fig. 57, carrying the engines and crew.

Consider the power required to drive this air ship at a given speed, Qf/s, or S m/h, against the resistance of skin friction of the air.

It is estimated as the result of experiment that frictional drag

1

is about of pressure due to normal impact, taken above at

2

400

(2) lb/ft2: so

20

2

Q
the friction is taken at (480)* lb/ft3.

Caution is required here not to take double or half this value; as experimenters have an awkward habit of tabulating frictional drag sometimes for both sides of the surface in their experiments. Here SX 50 × 500 = 78,540, say 80,000 ft2;

With screws working at 25 per cent. slip, their efficiency is 075, 75 %, so that S.H.P. 575, requiring probably 700 I.H.P. or more; or in a tabular form,

=

so that 360 I. H. P. should be expected to give a speed somewhere

between 30 and 36 m/h.

FINIS.