Page:The Solar System - Six Lectures - Lowell.djvu/54



over 12¾ miles deep, not the shallow, that would be nearest under the Moon.

Consider a body revolving freely around another in a circle, and disturbed in this motion by a third. This is the case with any particle of the ocean when we neglect pressure and friction. Connect the three bodies by lines, and, keeping their directions, increase their lengths inversely

as their squares, and join the ends. The disturbing force will be represented by the connecting line, on the principle of the composition of forces.

$$\textstyle \frac=\frac$$;

whence PN represents in amount and direction the disturbing or tide-raising force.