"That I understand."
"r is the radius of the earth."
"r, radius; admitted."
"m is the volume of the earth; m prime that of the moon. We are obliged to take into account the volume of the two attracting bodies, as the attraction is in proportion to the volume."
"I understand that."
"g represents gravity, the speed acquired at the end of a second by a body falling on the surface of the earth. Is that clear?"
"A mountain stream!" answered Michel.
"Now I represent by x the variable distance that separates the projectile from the centre of the earth, and by v the velocity the projectile has at that distance."
"Good."
"Lastly, the expression v zero which figures in the equation is the speed the bullet possesses when it emerges from the atmosphere."