We have time enough, however, to listen to another method before we get into the full blaze of the glorious old Sol." "The other method," interrupted M'Nicholl laying down his telescope to rest his eyes, and now joining in the conversation to give himself something to do, "is called that of the _tangent rays_.

A solar ray, barely passing the edge of the Moon's surface, is caught on the peak of a mountain the rest of which lies in shadow.

The distance between this starry peak and the line separating the light from the darkness, we measure carefully by means of our telescope.

Then--" "I see it at a glance!" interrupted Ardan with lighting eye; "the ray, being a tangent, of course makes right angles with the radius, which is known: consequently we have two sides and one angle--quite enough to find the other parts of the triangle.

Very ingenious--but now, that I think of it--is not this method absolutely impracticable for every mountain except those in the immediate neighborhood of the light and shadow line ?" "That's a defect easily remedied by patience," explained Barbican--the Captain, who did not like being interrupted, having withdrawn to his telescope--"As this line is continually changing, in course of time all the mountains must come near it.