Fig. 83 is, so to speak, a side view of the earth, showing the limit of sunshine on the earth when the sun is at the summer solstice; and Fig. 84, showing the limit of sunshine when the sun is at the autumnal equinox. [Illustration: Fig. 84.] 71. _Cause of the Change of Seasons._--Variety in the length of day and night, and diversity in the seasons, depend upon _the obliquity of the ecliptic_. Were there no obliquity of the ecliptic, there would be no inequality in the length of day and night, and but slight diversity of seasons. The greater the obliquity of the ecliptic, the greater would be the variation in the length of the days and nights, and the more extreme the changes of the seasons. Tides. 72. _Tides._--The alternate rise and fall of the surface of the sea twice in the course of a lunar day, or of twenty-four hours and fifty-one minutes, is known as the _tides_. When the water is rising, it is said to be _flood_ tide; and when it is falling, _ebb_ tide. When the water is at its greatest height, it is said to be _high_ water; and when at its least height, _low_ water. 73. _Cause of the Tides._--It has been known to seafaring nations from a remote antiquity that there is a singular connection between the ebb and flow of the tides and the diurnal motion of the moon. [Illustration: Fig. 85.] This tidal movement in seeming obedience to the moon was a mystery until the study of the law of gravitation showed it to be due to _the attraction of the moon on the waters of the ocean_. The reason why there are two tides a day will appear from Fig. 85. Let _M_ be the moon, _E_ the earth, and _EM_ the line joining their centres. Now, strictly speaking, the moon does not revolve around the earth any more than the earth around the moon; but the centre of each body moves around the common centre of gravity of the two bodies. The earth being eighty times as heavy as the moon, this centre is situated within the former, about three-quarters of the way from its centre to its surface, at the point _G_. The body of the earth itself being solid, every part of it, in consequence of the moon's attraction, may be considered as describing a circle once in a month, with a radius equal to _EG_. The centrifugal force caused by this rotation is just balanced by the mean attraction of the moon upon the earth. If this attraction were the same on every part of the earth, there would be everywhere an exact balance between it and the centrifugal force. But as we pass from _E_ to _D_ the attraction of the moon diminishes, owing to the increased distance: hence at _D_ the centrifugal force predominates, and the water therefore tends to move away from the centre _E_. As we pass from _E_ towards _C_, the attraction of the moon increases, and therefore exceeds the centrifugal force: consequently at _C_ there is a tendency to draw