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 guns in another couple of hours. The French claim that they have located new batteries, got their _tir de demolition_ upon them in and destroyed them within five hours. The British I told of that found it incredible. Every day the French print special maps showing the guns, sham guns, trenches, everything of significance behind the German lines, showing everything that has happened in the last four-and-twenty hours. It is pitiless. It is indecent. The map-making and printing goes on in the room next and most convenient to the examination of the photographs. And, as I say, the German army knows of this, and knows that it cannot prevent it because of its aerial weakness. That knowledge is not the last among the forces that is crumpling up the German resistance upon the Somme. I visited some French guns during the _tir de demolition_ phase. I counted nine aeroplanes and twenty-six kite balloons in the air at the same time. There was nothing German visible in the air at all. It is a case of eyes and no eyes.