one another, at the same time every day (that is supposing you are always in time for lunch); but since the tides depend on the Moon, the marks slope; and we could tell from the amount of slope that the marks referred to something depending on the

Moon,

Fig. 71.

because in a month they go right through the twenty-four hours and start afresh. When the marks form a long series like this, it is easy to interpret the diagram.

We must notice just one more thing: the line is not quite straight, but rather wavy; this is