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HISTORICAL VIEW OF THE

from it. The action of the sun is very small comparedwith that of the moon; hence, the high tide occursalmost exactly when the moon comes upon the meridian.This is highest when the sun, earth, and moon are inthe same line; and this, whether at opposition or con-junction, is called the spring tide. The action isweakest when the directions of the sun and moon are atright angles; this is the neap tide: in this, or at quadra-tures, the solar and lunar tides are opposed, or the highwater of the solar takes place at the same time as thelow water of the lunar, and conversely ; so that the ac-tual tide is the difference of the two : at opposition andconjunction it is the sum.

Another important subject in which the principle ofgravitation was found to be concerned, is the figure ofthe earth. The force of mutual attraction would aggre-gate the parts of matter together into a perfectly spheri-cal form. But if a rotatory motion were given to such asphere, it is an obvious consequence that — if its partshave in any degree a sufficiently yielding consistency toallow of it — the equatorial region will bulge out, thepolar parts will be flattened, and the whole assume thefigure of an oblate spheroid. Newton not only estab-lished this inference, but endeavoured to estimate whatits amount ought to be under given circumstances.This he did by means of the very ingenious illustrationof conceiving a pipe or canal reaching from the pole tothe centre of the earth, and another at right angles to it,opening from it and reaching from the centre to theequator, the polar and equatorial waters freely commu-nicating through it. The water in such a canal mustbe in equilibrio, or the weight of fluid in the shorterequal to that in the larger branch. Taking into con-sideration then the effect of centrifugal force, and theother more complicated conditions which were intro-duced by the spheroidal form, Newton calculated whatthe respective lengths of the canals ought to be; or, inother words, the proportion of the polar to the equato-rial radius of the earth. In this calculation, the earth