The others, laughing, told him that it was nothing serious, but Jack
looked a bit worried until the empty jacket fell on the grass and, a
little later, Tom himself came down smiling from aloft, all unaware of
the excitement he had caused.

CHAPTER XI. OVER THE LINES

"Well, I guess we stay downstairs, to-day," remarked Tom to Jack,
the day following their exhibition flights for the benefit of the air
students.

"Yes, it doesn't look very promising," returned his chum.

Jack looked aloft where the sky--or what took its place--was represented
by a gray mist that seemed ready to drip water at any moment. It was
a day of "low visibility," and one when air work was almost totally
suspended. This applied to the enemy as well as to the Yankees. For even
though it is feasible to go up in an aeroplane in fog, or even rain or
snow, it is not always safe to come down again in like conditions.

There is nothing worse than rain, snow or fog for clouding an aviator's
goggles, making it impossible for him to see more than a plane's length
ahead, if, indeed, he can see that far. Then, too, little, if anything,
can be accomplished by going aloft in a storm or fog. No observations
of any account can be made, and the aviator, once he gets aloft, is as
likely to come down behind the German lines as he is to descend safely
within his own.

That being the case, Tom and Jack, in common with their comrades of the
air, had a vacation period. Some of them obtained leave and went to the
nearest town, while some put in their time going over their guns and
glasses and equipment and machines.

Jack and Tom elected to do the latter. There was one very fast and
powerful Spad which they often used together, taking turns at piloting
it and acting as observer. They thought they might have a chance soon to
go over the German lines in this, their favorite craft, so they decided
to put in their spare time seeing that it was in perfect shape, and that
the two machine guns were ready for action when needed.

"'Would you rather do this than fly, Jack?" asked Tom, as they went
over, in detail, each part of the powerful Spad.

"I should say not! But, after all, one is just as important as the
other. I hope we get a good day to-morrow. I'd like to do something
toward seeing if we can't get Harry out of the Boche's clutches," and he
nodded in the direction of the German lines.

"'Tisn't going to be easy doing that," remarked Tom. "I'd ask noth