"What Delphic oracle says so ?" "Dear friend," replied Barbican quietly, "the visible mountains of the Moon have been measured so carefully and so accurately that I should hardly hesitate in affirming their altitude to be as well known as that of Mont Blanc, or, at least, as those of the chief peaks in the Himalayahs or the Rocky Mountain Range." "I should like to know how people set about it," observed Ardan incredulously.
"There are several well known methods of approaching this problem," replied Barbican; "and as these methods, though founded on different principles, bring us constantly to the same result, we may pretty safely conclude that our calculations are right.

We have no time, just now to draw diagrams, but, if I express myself clearly, you will no doubt easily catch the general principle." "Go ahead!" answered Ardan.

"Anything but Algebra." "We want no Algebra now," said Barbican, "It can't enable us to find principles, though it certainly enables us to apply them.Well.The Sun at a certain altitude shines on one side of a mountain and flings a shadow on the other.

The length of this shadow is easily found by means of a telescope, whose object glass is provided with a micrometer.

This consists simply of two parallel spider threads, one of which is stationary and the other movable.