the letter of the Cambridge Observatory had said all that was to
be said in this respect. Every one knew that this altitude
varies according to the latitude of the observer. But the only
zones of the globe in which the moon passes the zenith, that is,
the point directly over the head of the spectator, are of
necessity comprised between the twenty-eighth parallels and
the equator. Hence the importance of the advice to try the
experiment upon some point of that part of the globe, in order
that the projectile might be discharged perpendicularly, and so
the soonest escape the action of gravitation. This was an
essential condition to the success of the enterprise, and
continued actively to engage the public attention.

Regarding the path described by the moon in her revolution round
the earth, the Cambridge Observatory had demonstrated that this
path is a re-entering curve, not a perfect circle, but an
ellipse, of which the earth occupies one of the _foci_. It was
also well understood that it is farthest removed from the earth
during its _apogee_, and approaches most nearly to it at its _perigee_.

Such was then the extent of knowledge possessed by every
American on the subject, and of which no one could decently
profess ignorance. Still, while these principles were being
rapidly disseminated many errors and illusory fears proved less
easy to eradicate.

For instance, some worthy persons maintained that the moon was
an ancient comet which, in describing its elongated orbit round
the sun, happened to pass near the earth, and became confined
within her circle of attraction. These drawing-room astronomers