"Just enough for our purposes and no more," replied Frank, readily,
"fortunately the soluble tablets of picric and glycerine will help
out our supply materially. A few of these tablets dissolved in
gasoline render the efficiency of one ordinary gallon equal to
three; but I don't care to use them except in a case of absolute
necessity as they are very hard on an engine."

"Then we can count on every gallon we carry being of triple
efficiency?" asked Billy.

"Certainly," replied Frank, who had invented the tablets in
question, and which were an extremely useful addition to the
equipment of the modern aviator. As the boys worked on and the
equipment, as it was classified, was packed away in the cases
assigned to each class of articles, there came a sharp knock at the
door of the garage building and a servant entered with a special
delivery letter to Frank. The boy tore it open eagerly and then
gave a low whistle of astonishment.

"Read it out, Harry," he said, handing the missive to his brother.
"It concerns all of us."

Harry took it and read as follows:

DEAR FRANK AND HARRY:

Shall be in town to-morrow morning with my father and Mr. Luther
Barr, the well-known ivory importer. He has a communication of
importance for you. What it is I am afraid to trust to writing, but
you will know full details when you see us. Will you call at the