"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a defect easily remedied by patience," explained Barbican--the Captain, who did not like being interrupted, having withdrawn to his telescope--"As this line is continually changing, in course of time all the mountains must come near it. A third method--to measure the mountain profile directly by means of the micrometer--is evidently applicable only to altitudes lying exactly on the lunar rim." "That is clear enough," said Ardan, "and another point is also very clear. In Full Moon no measurement is possible. When no shadows are made, none can be measured. Measurements, right or wrong, are possible only when the solar rays strike the Moon's surface obliquely with regard to the observer. Am I right, Signor Barbicani, maestro illustrissimo?" "Perfectly right," replied Barbican. "You are an apt pupil." "Say that again," said Ardan. "I want Mac to hear it." Barbican humored him by repeating the observation, but M'Nicholl would only notice it by a grunt of doubtful meaning. "Was Galileo tolerably successful in his calculations?" asked Ardan, resuming the conversation. Before answering this question, Barbican unrolled the map of the Moon, which a faint light like that of day-break now enabled him to examine. He then went on: "Galileo was wonderfully successful--considering that the telescope which he employed was a poor instrument of his own construction, magnifying only thirty times. He gave the lunar mountains a height of about 26,000 feet--an altitude cut down by Hevelius, but almost doubled by Riccioli. Herschel was the first to come pretty close to the truth, but Beer and Maedler, whose _Mappa Selenographica_ now lies before us, have left really nothing more to be done for lunar astronomy--except, of course, to pay a personal visit to the Moon--which we have tried to do, but I fear with a very poor prospect of success." "Cheer up! cheer up!" cried Ardan. "It's not all over yet by long odds. Who can say what is still in store for us? Another bolide may shunt us off our ellipse and even send us to the Moon's surface." Then seeing Barbican shake his head ominously and his countenance become more and more depressed, this true friend tried to brighten him up a bit by feigning to take deep interest in a subject that to him was absolutely the driest in the world. "Meer and Baedler--I mean Beer and Maedler," he went on, "must have measured at least forty or fifty mountains to their satisfaction."