“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.” “Our curiosity will be gratified,” said Servadac, “if you will enlighten our ignorance.” But Rosette did not take the slightest notice of the interruption. “A substance it is which no doubt constitutes the sole material of the comet, extending from its surface to its innermost depths. The probability is that it would be so; your experience confirms that probability: you have found no trace of any other substance. Of this rock here is a solid decimeter; let us get at its weight, and we shall have the key which will unlock the problem of the whole weight of Gallia. We have demonstrated that the force of attraction here is only one-seventh of what it is upon the earth, and shall consequently have to multiply the apparent weight of our cube by seven, in order to ascertain its proper weight. Do you understand me, goggle-eyes?” This was addressed to Ben Zoof, who was staring hard at him. “No!” said Ben Zoof. “I thought not; it is of no use waiting for your puzzle-brains to make it out. I must talk to those who can understand.” The professor took the cube, and, on attaching it to the hook of the steelyard, found that its apparent weight was one kilogramme and four hundred and thirty grammes. “Here it is, gentlemen; one kilogramme, four hundred and thirty grammes. Multiply that by seven; the product is, as nearly as possible, ten kilogrammes. What, therefore, is our conclusion? Why, that the density of Gallia is just about double the density of the earth, which we know is only five kilogrammes to a cubic decimeter. Had it not been for this greater density, the attraction of Gallia would only have been one-fifteenth instead of one-seventh of the terrestrial attraction.” The professor could not refrain from exhibiting his gratification that, however inferior in volume, in density, at least, his comet had the advantage over the earth. Nothing further now remained than to apply the investigations thus finished to the determining of the mass or weight. This was a matter of little labor. “Let me see,” said the captain; “what is the force of gravity upon the various planets?” “You can’t mean, Servadac, that you have forgotten that? But you always were a disappointing pupil.” The captain could not help himself: he was forced to confess that his memory had failed him.