If we made a small model, with the sun in the center, and the earth and moon traveling a few inches off, only one slender piece of wire would be needed to represent the path of earth and moon together. For not only would the earth and the moon be so small as to be quite invisible, but the whole of the moon’s orbit would have disappeared into the thickness of the single wire. This question of the moon’s motions is in its nature intricate, and in its details quite beyond the grasp of any beginner in astronomy. But so much at least may be understood, that though the earth’s attraction powerfully affects the moon, and causes in her motions _perturbations_, such as have been already spoken about as taking place among the planets, yet that in reality the great controlling power over the moon is the attraction of the sun. The tides of the ocean are chiefly brought about by the moon’s attraction. The sun has something to do with the matter, but the moon is the chief agent. This action of the moon can best be seen in the southern hemisphere, where there is less land. As the moon travels slowly round the earth, her attraction draws up the yielding waters of the ocean in a vast wave which travels slowly along with her. The same pulling which thus lifts a wave on the side of the earth towards the moon, also pulls the earth gently _away from_ the water on the opposite side, and causes a second wave there. The parts of the ocean between these two huge waves are depressed, or lower in level. These two waves on the opposite sides of the earth sweep steadily onwards, following the moon’s movements,--not real, but seeming movements, caused by the turning of the earth upon her axis. Once in every twenty-four hours these wide waves sweep round the whole earth in the southern ocean. They can not do the same in the north, on account of the large continents, but offshoots from the south waves travel northwards, bringing high-tide into every sea and ocean inlet. If there were only one wave, there would be only one tide in each twenty-four hours. As there are two waves, there are two tides, one twelve hours after the other. In the space between these two high-tides we have low-tide. Twice every month we have very high and very low tides. Twice every month we have tides not so high or so low. The highest are called “spring-tides,” and the lowest “neap-tides.” When the moon is between us and the sun, or when she is “new moon,” there are spring-tides; for the pull or attraction of sun and moon upon the ocean act exactly together. It is the same at full moon, when once more the moon is in a straight line with earth and sun. But at the first and last quarters, when the moon has her _sideways_ position, and when the sun pulls in one direction and the moon pulls in another, each undoes a little of the other’s work. Then we only have neap-tides; for the wave raised is smaller, and the water does not flow so high upon our shores.