LECTURE VIII.

THE EARTH .

“ Age here on age

“ Lies heap’d like wither’d leaves. And must it end ? ”

Bailey.

In discussing the Earth as a member of the Solar System ,or in comparing it with the other planets which circle roundthe Sun, either within or without its orbit, it is almostimpossible to say where astronomy begins and geography andgeology end ; or, to use a term of somewhat recent invention,it is by no means easy to define the boundary-line betweenastronomy and physiography.

Some of the smallest variations in the Earth ’s movementsinvolve results of the deepest import in its past geologicalhistory, and in the interchange of glacial and torrid epochs.A deviation from perfect regularity in its shape, which mightat first sight seem of little moment, causes the precession ofthe equinoxes, and changes the whole aspect of the heavens, asthey are seen from any given terrestrial locality, in the courseof every 26,000 years. The temperature of a mine; the densityof the air at the highest altitude the aeronaut can attain;the speed with which a pendulum oscillates at the equator,or near to either pole, or in any intermediate latitude; thegreater accumulation of ice at the south than at the northpole; the varying duration of twilight; the beauties of theAurora; the Earth ’s ever-changing magnetic currents; thedisturbances of the action of telegraphic instruments; the rain-fall of any given year; the tides and currents of the ocean ;the fury of a cyclone ; the throbbing of an earthquake; theglowing fire of a volcano; the regulation of every watch and