s, and fight. In Sir John
French's dispatches mention is made of bomb-dropping from 3000 feet. In
these days the aerial battleground has been extended to anything up
to 20,000 feet. Indeed, so brisk has been the duel between gun and
aeroplane, that nowadays airmen have often to seek the other margin of
safety, and can defy the anti-aircraft guns only by flying so low as
just to escape the ground. The general armament of a "fighter" consists
of a maxim firing through the propeller, and a Lewis gun at the rear on
a revolving gun-ring.

It is pleasant to record that the Allies kept well ahead of the enemy in
their use of aerial photography. Before a great offensive some thousands
of photographs had to be taken of enemy dispositions by means of cameras
built into the aeroplanes.

Plates were found to stand the rough usage better than films, and not
for the first time in the history of mechanics the man beat the machine,
a skilful operator being found superior to the ingenious automatic
plate-fillers which had been devised.

The counter-measure to this ruthless exposure of plans was camouflage.
As if by magic-tents, huts, dumps, guns began, as it were, to sink into
the scenery. The magicians were men skilled in the use of brush and
paint-pot, and several leading figures in the world of art lent their
services to the military authorities as directors of this campaign of
concealment. In this connection it is interesting to note that both
Admiralty and War Office took measures to record the pictorial side
of the Great War. Special commissions were given to a notable band of
artists working in their different "lines". An abiding record of the
great struggle will be afforded by the black-and-white work of Muirhead
Bone, James M'Bey, and Charles Pears; the portraits, landscapes, and
seascapes of Sir John Lavery, Philip Connard, Norman Wilkinson, and
Augustus John, who received his commission from the Canadian Government.

CHAPTER XL. The Atmosphere and the Barometer

For the discovery of how to find the atmospheric pressure we are
indebted to an Italian named Torricelli, a pupil of Galileo, who carried
out numerous experiments on the atmosphere toward the close of the
sixteenth century.

Torricelli argued that, as air is a fluid, if it had weight it could
be made to balance another fluid of known weight. In his experiments he
found that if a glass tube about 3 feet in length, open at one end
only, and filled with mercur