voice," she explained. "Listen!" and again she sang.
"I do not understand," he insisted; "but I like it. Could you teach me to do it?"
"I do not know, but I shall be glad to try."
"We will see what Luud does with you," he said. "If he does not want you I will keep you and you shall teach me to make sounds like that."
At his request she sang again as they continued their way along the winding tunnel, which was now lighted by occasional bulbs which appeared to be similar to the radium bulbs with which she was familiar and which were common to all the nations of Barsoom, insofar as she knew, having been perfected at so remote a period that their very origin was lost in antiquity. They consist, usually, of a hemispherical bowl of heavy glass in which is packed a compound containing what, according to John Carter, must be radium. The bowl is then cemented into a metal plate with a heavily insulated back and the whole affair set in the masonry of wall or ceiling as desired, where it gives off light of greater or less intensity, according to the composition of the filling material, for an almost incalculable period of time.
As they proceeded they met a greater number of the inhabitants of this underground world, and the girl noted that among many of these the metal and harness were more ornate than had been those of
