of the special Krupp anti-aircraft field-piece, exclusive
of the protecting shield, is approximately identical with that of the
ordinary light artillery field-piece. It throws a shell weighing 8.8
pounds with an initial velocity of about 2,066 feet per second.

Although the German armament manufacturers were among the first to enter
the field with an anti-aircraft gun of this character they were speedily
followed by the French, who devised a superior weapon. In fact, the
latter represented such a decisive advance that the German artillerists
did not hesitate to appropriate their improvements in sundry essential
details, and to incorporate them with their own weapons. This applies
especially to the differential recoil system which is utilised in the
small anti-aircraft guns now mounted upon the roofs of high buildings
of cities throughout Germany for the express purpose of repelling aerial
attack.

The French system is admitted by the leading artillery technicians of
the world to be the finest which has ever been designed, its remarkable
success being due to the fact that it takes advantage of the laws of
Nature. In this system the gun is drawn back upon its cradle preparatory
to firing. In some instances the barrel is compressed against a spring,
but in the more modern guns it is forced to rest against a cushion of
compressed air contained within a cylinder. When first bringing the
gun into action, the barrel is brought into the preliminary position by
manually compressing the air or spring by means of a lever. Thereafter
the gun works automatically. When the gun is fired the barrel is
released and it flies forward. At a critical point in its forward travel
the charge is fired and the projectile speeds on its way. The kick or
recoil serves to arrest the forward movement of the barrel and finally
drives it back again against the strong spring or cushion of compressed
air within the cylinder to its normal position, when it is ready for the
introduction of the next shell.

The outstanding feature of this system is that the projectile is given
a higher initial velocity than is possible with the barrel held rigid at
the moment of discharge, because the shell is already travelling at the
moment of firing.

The fixed anti-aircraft guns such as are stationed upon eminences and
buildings are of the quick firing type, the object being to hurl
a steady, continuous stream of missiles upon the swiftly moving
aeroplane. Some of