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29 Moon. It is calculated that the Moon can raise the Waters of the ocean 6 feet above their mean level, and the Sun 2 feet. At new Moon, the Sun and Moon will he both acting in the same direction; and the water will rise 8 feet instead of 6, being what is termed a spring tide. The same thing happens at full Moon, because each of these bodies acts on the same parts of the Earth at once ; and they thus raise the tide nearly to the height it would he at new moon. But when the Moon is at the quarter, the Sun acts on those parts where, by the Moon’s influence, there would be low water. Their attractions are therefore opposed to each other; and the water neither rises so high nor falls so low, as at other times. This is called neap tide.* The rise and fall of tide in various places is very much modified, both in time and quantity, by various local circumstan- ces ; it is therefore impossible to ascertain by mere calculation the state of the tide at any place, except in the open sea. Having now considered the principal phenomena of the Solar System, we shall explain some of their causes. It has been already stated that the planets move round the Sun, and the satellites round the planets, in cliptical orbits; and it will now be shown how these motions are produced by the joint action of two forces, " the tendency to continuous motion in a straight line, ■—and attraction towards a central body. DIAGRAM XXIV. In this figure, E represents the Earth, which we may conceive to have been projected through space in the direction E A. Now, if no other force operated Upon it, this body would continue to move onwards in the same straight line towards 1 and 2. On the other hand, suppose the Earth to be situated at A, and to . * These different effects are well shown by the moveable slide, No. 3 Ui the other set. C 2