Ben Zoof looked on in perplexity, regarding the lecturer with much the same curiosity as he would have watched the performances of a traveling mountebank at a fair in Montmartre; but Servadac and his two friends had already divined the professor’s meaning. They knew that French coinage is all decimal, the franc being the standard of which the other coins, whether gold, silver, or copper, are multiples or measures; they knew, too, that the caliber or diameter of each piece of money is rigorously determined by law, and that the diameters of the silver coins representing five francs, two francs, and fifty centimes measure thirty-seven, twenty-seven, and eighteen millimeters respectively; and they accordingly guessed that Professor Rosette had conceived the plan of placing such a number of these coins in juxtaposition that the length of their united diameters should measure exactly the thousand millimeters that make up the terrestrial meter.
The measurement thus obtained was by means of a pair of compasses divided accurately into ten equal portions, or decimeters, each of course 3.93 inches long. A lath was then cut of this exact length and given to the engineer of the Dobryna, who was directed to cut out of the solid rock the cubic decimeter required by the professor.
The next business was to obtain the precise weight of a kilogramme. This was by no means a difficult matter. Not only the diameters, but also the weights, of the French coins are rigidly determined by law, and as the silver five-franc pieces always weigh exactly twenty-five grammes, the united weight of forty of these coins is known to amount to one kilogramme.
“Oh!” cried Ben Zoof; “to be able to do all this I see you must be rich as well as learned.”
With a good-natured laugh at the orderly’s remark, the meeting adjourned for a few hours. By the appointed time the engineer had finished his task, and with all due care had prepared a cubic decimeter of the material of the comet.
“Now, gentlemen,” said Professor Rosette, “we are in a position to complete our calculation; we can now arrive at Gallia’s attraction, density, and mass.”
Everyone gave him his complete attention.
“Before I proceed,” he resumed, “I must recall to your minds Newton’s general law, ‘that the attraction of two bodies is directly proportional to the product of their masses, and inversely proportional to the square of their distances.'”
“Yes,” said Servadac; “we remember that.”
“Well, then,” continued the professor, “keep it in mind for a few minutes now. Look here! In this bag are forty five-franc pieces— altogether they weigh exactly a kilogramme; by which I mean that if we were on the earth, and I were to hang the bag on the hook of the steelyard, the indicator on the dial would register one kilogramme. This is clear enough, I suppose?”
As he spoke the professor designedly kept his eyes fixed upon Ben Zoof. He was avowedly following the example of Arago, who was accustomed always in lecturing to watch the countenance of the least intelligent of his audience, and when he felt that he had made his meaning clear to him, he concluded that he must have succeeded with all the rest. In this case, however, it was technical ignorance, rather than any lack of intelligence, that justified the selection of the orderly for this special attention.
Satisfied with his scrutiny of Ben Zoof’s face, the professor went on. “And now, gentlemen, we have to see what these coins weigh here upon Gallia.”
He suspended the money bag to the hook; the needle oscillated, and stopped. “Read it off!” he said.
The weight registered was one hundred and thirty-three grammes.
“There, gentlemen, one hundred and thirty-three grammes! Less than one-seventh of a kilogramme! You see, consequently, that the force of gravity here on Gallia is not one-seventh of what it is upon the earth!”
“Interesting!” cried Servadac, “most interesting! But let us go on and compute the mass.”
“No, captain, the density first,” said Rosette.
“Certainly,” said the lieutenant; “for, as we already know the volume, we can determine the mass as soon as we have ascertained the density.”
The professor took up the cube of rock. “You know what this is,” he went on to say. “You know, gentlemen, that this block is a cube hewn from the substance of which everywhere, all throughout your voyage of circumnavigation, you found Gallia to be composed— a substance to which your geological attainments did not suffice to assign a name.”