Page:Popular Astronomy - Airy - 1881.djvu/12

 x that the instrumental conceptions derived from the use of a common globe are sufficient, in almost every case, for the understanding of the instruments in an Observatory; that the elements which are the subjects of observation with Astronomical instruments, are the same as those with which we are most familiar in the ordinary globe-problems; and that a person who understands the latter can proceed at once with the former.

Afterwards, I have endeavoured to explain that the methods used for measuring Astronomical distances are in some applications absolutely the same as the methods of ordinary theodolite-surveying, and are in other applications equivalent to them; and that in fact there is nothing in their principles which will present the smallest difficulty to a person who has attempted the common operation of plotting from angular measures.

The elucidation of the theory of centripetal and disturbing forces is necessarily less complete. Still it appears probable that a general conception of the nature of the action of those forces, perfectly accurate as far as it goes, and sufficient to preserve the student from the gross errors into which many persons have fallen, may be obtained from explanations like those here offered.

The methods of ascertaining the weight of the Earth and other bodies, are still more difficult of explanation; yet it is hoped that something may be done even in these.

The first conclusion, then, which I would desire to impress upon the student of Astronomy, who enters upon the science with a tolerable understanding of ordinary practical matters, is, that nothing is totally beyond his reach. Complete