for the examination of the photographs. Both British and French air
photographs are thoroughly scrutinised and marked.

An air photograph to an inexperienced eye is not a very illuminating
thing; one makes our roads, blurs of wood, and rather vague buildings.
But the examiner has an eye that has been in training; he is a picked
man; he has at hand yesterday's photographs and last week's photographs,
marked maps and all sorts of aids and records. If he is a Frenchman he
is only too happy to explain his ideas and methods. Here, he will point
out, is a little difference between the German trench beyond the wood
since yesterday. For a number of reasons he thinks that will be a new
machine gun emplacement; here at the centre of the farm wall they have
been making another. This battery here--isn't it plain? Well, it's a
dummy. The grass in front of it hasn't been scorched, and there's been
no serious wear on the road here for a week. Presently the Germans will
send one or two waggons up and down that road and instruct them to make
figures of eight to imitate scorching on the grass in front of the gun.
We know all about that. The real wear on the road, compare this and this
and this, ends here at this spot. It turns off into the wood. There's a
sort of track in the trees. Now look where the trees are just a little
displaced! (This lens is rather better for that.) _That's_ one gun. You
see? Here, I will show you another....

That process goes on two or three miles behind the front line. Very
clean young men in white overalls do it as if it were a labour of love.
And the Germans in the trenches, the German gunners, _know it is going
on._ They know that in the quickest possible way these observations of
the aeroplane that was over them just now will go to the gunners. The
careful gunner, firing by the map and marking by aeroplane, kite balloon
or direct observation, will be getting onto the located guns and machine