"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. 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a defect easily remedied by patience," explained Barbican—the Captain, who did not like being interrupted, having withdrawn to his telescope—"As this line is continually changing, in course of time all the mountains must come near it. 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."
"Forty or fifty!" exclaimed Barbican. "They measured no fewer than a thousand and ninety-five lunar mountains and crater summits with a perfect success. Six of these reach an altitude of upwards of 18,000 feet, and twenty-two are more than 15,000 feet high."
"Which is the highest in the lot?" asked Ardan, keenly relishing Barbican's earnestness.
"Doerfel in the southern hemisphere, the peak of which I have just pointed out, is the highest of the lunar mountains so far measured," replied Barbican. "It is nearly 25,000 feet high."
"Indeed! Five thousand feet lower than Mount Everest—still for a lunar mountain, it is quite a respectable altitude."
"Respectable! Why it's an enormous altitude, my dear friend, if you compare it with the Moon's diameter. The Earth's diameter being more than 3-1/2 times greater than the Moon's, if the Earth's mountains bore the same ratio to those of the Moon, Everest should be more than sixteen miles high, whereas it is not quite six."
"How do the general heights of the Himalayahs compare with those of the highest lunar mountains?" asked Ardan, wondering what would be his next question.
"Fifteen peaks in the eastern or higher division of the Himalayahs, are higher than the loftiest lunar peaks," replied Barbican. "Even in the western, or lower section of the Himalayahs, some of the peaks exceed Doerfel."
"Which are the chief lunar mountains that exceed Mont Blanc in altitude?" asked Ardan, bravely suppressing a yawn.
"The following dozen, ranged, if my memory does not fail me, in the exact order of their respective heights;" replied Barbican, never wearied in answering such questions: "Newton, Curtius, Casatus, Rheita, Short, Huyghens, Biancanus, Tycho, Kircher, Clavius, Endymion, and Catharina."
"Now those not quite up to Mont Blanc?" asked Ardan, hardly knowing what to say.
"Here they are, about half a dozen of them: Moretus, Theophilus, Harpalus, Eratosthenes, Werner, and Piccolomini," answered Barbican as ready as a schoolboy reciting his lesson, and pointing them out on the map as quickly as a compositor distributing his type.
"The next in rank?" asked Ardan, astounded at his friend's wonderful memory.
"The next in rank," replied Barbican promptly, "are those about the size of the Matterhorn, that is to say about 2-3/4 miles in height. They are Macrobius, Delambre, and Conon. Come," he added, seeing Ardan hesitating and at a loss what other question to ask, "don't you want to know what lunar mountains are about the same height as the Peak of Teneriffe? or as Ætna? or as Mount Washington? You need not be afraid of puzzling me. I studied up the subject thoroughly, and therefore know all about it."
"Oh! I could listen to you with delight all day long!" cried Ardan, enthusiastically, though with some embarrassment, for he felt a twinge of conscience in acting so falsely towards his beloved friend. "The fact is," he went on, "such a rational conversation as the present, on such an absorbing subject, with such a perfect master—"
"The Sun!" cried M'Nicholl starting up and cheering. "He's cleared the disc completely, and he's now himself again! Long life to him! Hurrah!"
"Hurrah!" cried the others quite as enthusiastically (Ardan did not seem a bit desirous to finish his sentence).
They tossed their maps aside and hastened to the window.
Chapter XVII.
Tycho.
It was now exactly six o'clock in the evening. The Sun, completely clear of all contact with the lunar disc, steeped the whole Projectile in his golden rays. The travellers,