m by a time longer in proportion as
the distance to be travelled is greater.

There are objects in the heavens so distant that it would take many
hundreds of thousands of years for their light to reach us. Then it
necessarily follows, since we can see them, that they must have been
created and must have been shining so long.

The velocity with which light moves was first determined by the Danish
astronomer Roemer from the eclipses of Jupiter's satellites, November,
1675. It was, therefore, a determination of the rate for reflected solar
light in a vacuum, and gave 198,000 miles in a second. In 1727, Bradley
determined it for direct stellar light by his great discovery of the
aberration of the fixed stars. More recently, the experiments of M.
Foucault and those of M. Fizeau, by the aid of rotating mirrors or
wheels, have confirmed these astronomical observations, Fizeau's
determination of the velocity approaching that of Roemer. Probably,
however, the most correct is that of Struve, 191,515 miles per second.

[Sidenote: Investigation of the age of the earth through the phenomena
of heat.] This astronomical argument, which serves as a general
introduction, is strengthened by numerous physical and physiological
facts. But of the different methods by which the age of the earth may be
elucidated, I shall prefer that which approaches it through the
phenomena of heat. Such a manner of viewing the problem has led to its
determination in the minds of many thinking men.

[Sidenote: Astronomical heat alone on the earth's surface.] As correct
astronomical ideas began to prevail, it was perceived that all the heat
now on the surface of our planet is derived from the sun. Through the
circumstance of the inclination of her axis of rotation to the plane of
her annual motion, or through the fact of her globular form occasioning
the presentation of different parts of her surface, according to their
latitudes, with more or less obliquity, and hence the reception of less
or more of the rays, there may be local and temporary variations. But
these do not affect the general principle that the quantity of heat thus
received must be the same from year to year.

[Sidenote: The equilibrium of interior heat.] This thermometric
equilibrium not only holds good for the surface, it may also be
demonstrated for the whole mass of the planet. The day has not shortened
by the 1/200 of a second since the time of Hipparchus, and therefore the
decrease