The Moon's real diameter being known and occupying a certain space on the object glass, the exact space occupied by the shadow can be easily ascertained by means of the movable thread.

This space, compared with the Moon's space, will give us the length of the shadow.

Now, as under the same circumstances a certain height can cast only a certain shadow, of course a knowledge of the one must give you that of the other, and _vice versa_.

This method, stated roughly, was that followed by Galileo, and, in our own day, by Beer and Maedler, with extraordinary success." "I certainly see some sense in this method," said Ardan, "if they took extraordinary pains to observe correctly.

The least carelessness would set them wrong, not only by feet but by miles.