"That I comprehend."

"_m_ is the mass or volume of the Earth; _m_ prime that of the Moon. We
must take the mass of the two attracting bodies into consideration,
since attraction is in direct proportion to their masses."

"That I comprehend."

"_g_ is the gravity or the velocity acquired at the end of a second by a
body falling towards the centre of the Earth. Clear?"

"That I comprehend."

"Now I represent by _x_ the varying distance that separates the
Projectile from the centre of the Earth, and by _v_ prime its velocity
at that distance."

"That I comprehend."

"Finally, _v_ is its velocity when quitting our atmosphere."

"Yes," chimed in the Captain, "it is for this point, you see, that the
velocity had to be calculated, because we know already that the initial
velocity is exactly the three halves of the velocity when the Projectile
quits the atmosphere."

"That I don't comprehend," cried the Frenchman, energetically.

"It's simple enough, however," said Barbican.