“We regret,” he said seriously, “that at the moment there is no aeroplane in sight.  We will, however, show Madame everything.”

He led the way round the corner of the building to where a path, neatly banked, went out through the mud to the battery.

“Keep to the path,” said a tall sign.  But there was no temptation to do otherwise.  There must have been fifty acres to that field, unbroken by hedge or tree.  As we walked out, Captain Mignot paused and pointed his finger up and somewhat to the right.

“German shrapnel!” he said.  True enough, little spherical clouds told where it had burst harmlessly.

As cannonading had been going on steadily all the afternoon, no one paid any particular attention.  We walked on in the general direction of the trenches.

The gunners were playing prisoner’s base just beyond the guns.  When they saw us coming the game ceased, and they hurried to their stations.  Boys they were, most of them.  The youth of the French troops had not impressed me so forcibly as had the boyishness of the English and the Belgians.  They are not so young, on an average, I believe.  But also the deception of maturity is caused by a general indifference to shaving while in the field.

But Captain Mignot evidently had his own ideas of military smartness, and these lads were all clean-shaven.  They trooped in from their game, under that little cloud of shrapnel smoke that still hung in the sky, for all the world a crowd of overheated and self-conscious schoolboys receiving an unexpected visit from the master of the school.

The path ended at the battery.  In the centre of the guns was a raised platform of wood, and a small shelter house for the observer or officer on duty.  There were five guns in pits round this focal point and forming a circle.  And on the platform in the centre was a curious instrument on a tripod.

“The telemeter,” explained Captain Mignot; “for obtaining the altitude of the enemy’s aeroplane.”

Once again we all scanned the sky anxiously, but uselessly.

“I don’t care to have any one hurt,” I said; “but if a plane is coming I wish it would come now.  Or a Zeppelin.”

The captain’s serious face lighted in a smile.

“A Zeppelin!” he said.  “We would with pleasure wait all the night for a Zeppelin!”

He glanced round at the guns.  Every gunner was in his place.  We were to have a drill.

“We will suppose,” he said, “that a German aeroplane is approaching.  To fire correctly we must first know its altitude.  So we discover that with this.”  He placed his hand on the telemeter.  “There are, you observe, two apertures, one for each eye.  In one the aeroplane is seen right side up.  In the other the image is inverted, upside down.  Now!  By this screw the images are made to approach, until one is superimposed exactly over the other.  Immediately on the lighted dial beneath is shown the altitude, in metres.”