igns of anti-aircraft motor-driven
guns--"Archibalds" the British airmen term them with emphatic levity.
They are sturdily-built vehicles fitted with heavy motors, developing
from 40 to 50 horse-power, with the chassis not widely dissimilar from
that adopted for motor-omnibus traffic. Consequently, they are not
necessarily condemned to the high-roads, but within certain limits
are able to travel across country, i.e., upon fields or other level
expanses, where the soil is not unduly soft.

But the very character of the problem rendered the evolution of the
vehicle a somewhat perplexing matter. There were many factors which had
to be taken into consideration, and it was possible to meet the imposed
requirements only within certain limits. In the first place, the weight
of the gun itself had to be kept down. It was obviously useless to
overload the chassis. Again, the weight of the projectile and its
velocity had to be borne in mind. A high velocity was imperative.
Accordingly, an initial velocity varying from 2,200 to 2,700 feet per
second, according to the calibre of the gun, was determined.

Moreover, as mobility was an indispensable condition, the gun had to be
so mounted that it could be fired from the motor-car even if the
latter were travelling at high speed. This requirement entailed another
difficulty. The gun had to be mounted in such a manner as to enable the
gunner to train it easily and readily through the complete circle and
through its complete range of vertical inclination. As the result
of prolonged experiments it was ascertained that the most suitable
arrangement was a pedestal mounting, either within a turret or upon an
open deck. To meet the weight of the gun, as well as the strains and
stresses incidental to firing, the chassis was strengthened, especially
over the rear axle near which the mounting is placed.

The heaviest gun of this type is the 10.5 centimetre (4 1/4-inch)
quick-firer, throwing a shell weighing nearly forty pounds, with an
initial velocity of 2,333 feet per second. This "Archibald" is totally
unprotected. The gun is mounted centrally upon the carriage over the
rear axle, and occupies the centre of the deck between the driver's seat
and that of the gun crew behind. The whole of the deck is clear, thereby
offering no obstruction to the gunner in training the weapon, while the
space may be widened by dropping down the wings of the vehicle. At the
rear is a seat to accommodate the gun