. When she had finished he turned toward her again.

"That was different," he said. "I liked that better, even, than the
other. How do you do it?"

"Why," she said, "it is singing. Do you not know what song is?"

"No," he replied. "Tell me how you do it."

"It is difficult to explain," she told him, "since any explanation of
it presupposes some knowledge of melody and of music, while your very
question indicates that you have no knowledge of either."

"No," he said, "I do not know what you are talking about; but tell me
how you do it."

"It is merely the melodious modulations of my 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 the workers
in the fields above. The heads and bodies, however, were similar, even
identical, she thought. No one offered her harm and she was now
experiencing a feeling of relief almost akin to happiness, when her
guide turned suddenly into an opening on the right side of the tunnel
and she found herself in a large, well lighted chamber.

CHAPTER V

THE PERFECT BRAIN

The song that had been upon her lips as she entered died there--frozen
by the sight of horror that met her eyes. In the center of the chamber
a