"Where?" exclaimed Ned again, sharply.
"Where?" echoed Alan.
"Why, at the sea level-that's where. Not out here. Do you know how high we are above sea level right here?"
Alan began to see the point and a smile came over his face. He had no chance to answer:
"We're a little short of seven thousand feet up in the air right here in Clarkeville," continued Ned in about the same tone of exultation he might have used had he found a gold mine. "Now, listen. How many cubic feet of gas does our balloon hold?"
That question was easy. The boys knew that as well as the multiplication table.
"Sixty-five thousand, four hundred and ninety-three feet."
"And how much weight is it going to carry?"
"Three thousand nine hundred and thirty-five and a half pounds."
"Exactly," went on Ned. "That's the weight we are going to carry figured at sea level. Did it ever occur to you that our sixty-five hundred feet of hydrogen can lift more way up here seven thousand feet in the air, than it can at sea level? Did it ever occur to my special engineer and calculator that as the weight and pressure of the air grows less our hydrogen will lift just that much more weight.