"Then you're just as deep a one as our President."

"No, Ardan; not at all. The really difficult part of the question
Barbican has done. That is, to make out such an equation as takes into
account all the conditions of the problem. After that, it's a simple
affair of Arithmetic, requiring only a knowledge of the four rules to
work it out."

"Very simple," observed Ardan, who always got muddled at any kind of a
difficult sum in addition.

"Captain," said Barbican, "_you_ could have found the formulas too, if
you tried."

"I don't know about that," was the Captain's reply, "but I do know that
this formula is wonderfully come at."

"Now, Ardan, listen a moment," said Barbican, "and you will see what
sense there is in all these letters."

"I listen," sighed Ardan with the resignation of a martyr.

"_d_ is the distance from the centre of the Earth to the centre of the
Moon, for it is from the centres that we must calculate the
attractions."

"That I comprehend."

"_r_ is the radius of the Earth."