nd it is in this subject that the airship possesses such
undoubted advantage over the aeroplane. In the heavier-than-air
machine there is no automatic improvement in efficiency resulting from
greater dimensions. In the airship, however, this automatic
improvement takes place in a very marked degree; for example, an
airship of 10,000,000 cubic feet capacity has five times the lift of
the present 2,000,000 cubic feet capacity rigid, but the length of the
former is only 1.7 times greater, and therefore the weight of the
structure only five times greater (1.7); that is, the weight of the
structure is directly proportional to the total lift. Having seen that
the total lift varies as the cube of the linear dimensions while air
resistance, B.H.P.--other things being equal--vary as the square of the
linear dimensions, it follows that the ratio "weight of machinery/total
lift" decreases automatically.

In comparing the different methods of transport for efficiency, the
resistance or thrust required is compared as a percentage of the total
weight. The result obtained is known as the "co-efficient of tractive
resistance." Experiments have shown that as the size of the airship
increases, the co-efficient of tractive resistance decreases to a
marked extent; with a proportionate increase in horse-power it is
proportionally more economical for a 10,000,000 cubic feet capacity
rigid to fly at 80 miles per hour than for a 2,000,000 cubic feet
capacity to fly at 60 miles per hour.

As the ratio "weight structure/total lift" is in airships fairly
constant, it follows that the ratio "disposable lift/total lift"
increases with the dimensions.

It is therefore obvious that increased benefits are obtained by
building airships of a larger size, and that the bigger the ship the
greater will be its efficiency, providing, of course, that it is kept
within such limits that it can be handled on the ground and manoeuvred
in the air.

The proportion of the useful lift in a large rigid, that is the lift
available for fuel, crew, passengers, and merchandise, is well over 50
per cent when compared with the gross lift. When the accompanying table
is studied it will be seen that with airships of large capacity the
available lift will be such that considerable weights of merchandise or
passengers can be carried.

Capacity in Gross Lift Length Diameter
cubic feet in tons in feet in feet
2