A tree judged by the height of a human being gives, at all events, a standard for a mountain; and, supposing this is, say, a mile high, it can serve as unit for the number expressing the earth's diameter, so as to make it intuitable; similarly the earth's diameter for the known planetary system; this again for the system of the Milky Way; and the immeasurable host of such systems, which go by the name of nebulae, and most likely in turn themselves form such a system, holds out no prospect of a limit.