Again, the difference between high and low water marks is not always exactly the same for the same kind of tide--the spring tide for a certain period, for example, not having the same limits as the same tide of another time. This is due to the fact that the moon revolves round the sun in an elliptical orbit, while the earth, at the same time, revolves round the sun in a similar path, so that the distances of both moon and sun from the earth vary at different times. And, since the tide-producing influences of both these bodies must increase as their distance from the earth diminishes, it follows that there must be occasional appreciable variations in the vigour of the tidal movements of the ocean. As the earth rotates on its axis, while at the same time the tidal wave must necessarily keep its position under the moon, this wave appears to sweep round the earth with considerable velocity. The differences in the level of the ocean thus produced would hardly be appreciable if the earth were entirely covered with water; but, owing to the very irregular distribution of the land, the movements of the tidal wave become exceedingly complex; and, when it breaks an entrance into a gradually narrowing channel, the water is compressed laterally, and correspondingly increased in height. It is thus that we find a much greater difference between the levels of high and low tides in continental seas than are to be observed on the shores of oceanic islands. We have occupied so much of our time and space in explanation of the movements of the tides not only because we think it desirable that all who delight in sea-side rambles should understand something of the varied motions which help to give such a charm to the sea, but also because, as we shall observe later, these motions are a matter of great importance to those who are interested in the observation and study of marine life. And, seeing that we are writing more particularly for the young naturalists of our own island, we must devote a little space to the study of the movements of the tidal wave round Great Britain, in order that we may understand the great diversity in the time of high tide on any one day on different parts of the coast, and see how the time of high tide for one part may be calculated from that of any other locality. Were it not for the inertia of the ocean and the resistance offered by the irregular continents, high tide would always exist exactly under the moon, and we should have high water at any place just at the time when the moon is in the south and crossing the meridian of that place. But while the inertia of the water tends to make all tides late, the irregular distribution of the land breaks up the tidal wave into so many wave-crests and greatly retards their progress.