If this velocity could be increased to something over
four miles per second, the path of the bullet would be



The Motions of the Planets 113

nearly four thousand miles long, the bullet coming to
the earth at this distance from the tower. If the
muzzle velocity were increased beyond this point to
about 4.9 miles per second, then the shell would miss the
earth entirely and would travel about the earth in a
circle. As the circumference of the earth is approxi-
mately 25,000 miles, it would take such a projectile
travelling 4.9 miles each second about 5100 seconds, or
I hour 24.7 minutes, to complete one circuit about the
earth and to return to the starting point.
If the muzzle velocity were still further increased,
the path of the projectile would become an ellipse and
its period would become longer. When the velocity
reached 6.94 miles per second, the path would become
a parabola and the bullet would never return to the
starting point.
