If, however, they merely thought that I had escaped, and was not invisible, they would feel that they need only search for me to find me very quickly. Of course, they might still place a guard over the flier; but such a guard would not be so on the alert, and we still might board the ship and get away before they were aware of our presence.

As we approached the first courtyard, I suddenly broke away from my guard and ran ahead with all my earthly speed. The warrior shouted for me to halt, and broke into a run. As I reached the entrance to the courtyard I pretended to dodge around the corner, which would of course have hidden me from him.

I must confess that in that short sprint my heart had been in my mouth, for of course I could not know whether or not I should become invisible.

However, the moment that I left the lighted corridor I absolutely disappeared; I could not see any part of my body-it was the strangest sensation that I have ever experienced.

I had made my plans, and now I ran to the far end of the courtyard and leaped lightly to the roof of the city.

I could hear the warrior guard rushing about calling to me; my disappearance must certainly have mystified him, for having no idea that I could become invisible, there was really no way in which he could account for it except on the theory that I had run into the entrance to another street. However, he was probably confident that I did not have time to do this.

Well, I did not bother much about him or what he was thinking; instead I took off across the roof in search of the courtyard where Ptor Fak was awaiting me and where I expected to meet Rojas at midnight; and it was pretty close to what we call midnight then, the Barsoomian midnight occurring twenty-five xats after the eighth zode.

A Martian day is divided into ten zodes, there being four tals to a xat, or two hundred to a zode. The dials of their clocks are marked with four concentric circles; between the inner circle and the next outer one the zodes are marked from one to ten; in the next circle, the xats are marked from one to fifty between each two zodes; and in the outer circle two hundred tals are marked between the radii which pass through the zode numbers and extend to the outer periphery of the dial. Their clock has three different colored and different length hands, one indicating the zode, the second one the xat, and the longest one the tal.

(Editor's note: I have before me the diagram of the dial of a Martian clock drawn for me by John Carter many years ago.) I had no difficulty in finding the courtyard in which I had been confined; and when I reached it I whistled, and Ptor Fak answered. I dropped down into it and whistled again, and when Ptor Fak answered I groped around until I bumped into him.

"How well you look," he said, and we both laughed. "It took you much longer to dispose of Motus than I had anticipated," he continued.

"I had to drag it out so that I would be sure to be invisible when I had returned here," I explained.

"And now what?" asked Ptor Fak.

I found his head and placed my lips close to one of his ears. "After Rojas comes," I whispered, "we'll cross the roof to the quarters of the slave women and get Llana of Gathol. In the meantime, you climb this tree which overhangs the roof and wait for us up there."

"Whistle when you come up," he said, and left me.

Invisibility I discovered was most disconcerting; I could see no part of my body; I was only a voice without visible substance-a voice standing in an apparently deserted courtyard which might be filled with enemies, as far as I knew. I couldn't even have heard them had there been any there, for the Invaks have taken the precaution of covering all the metal parts of their accouterments so that there is not the usual clank of metal upon metal when they move about.

Knowing as I did that a search for me must have been instituted, I felt positive that there must be Invak warriors in the courtyard, notwithstanding the fact that I neither heard nor saw anyone.

As I waited for Rojas, I took the precaution of not moving about lest I inadvertently bump into someone who might require me to identify myself; but I could not prevent someone from bumping into me, and that is exactly what happened. Hands were laid upon me and a gruff voice demanded, "Who are you?"

Here was a pretty kettle of fish. What was I to do? I doubted that I could pass myself off as an Invak-I knew too little about them to do that successfully; so, I did the next best thing that occurred to me.

"I am the ghost of Motus," I said, in a sepulchral voice. "I am searching for the man who killed me, but he is not here."

The hands relinquished their hold upon me; I could almost feel the fellow shrink away from me, and then another voice said, "Ghost of Motus nothing-I recognize that voice-it is the voice of the slave who killed Motus. Seize him!"

I jumped to one side but I jumped into the arms of another voice, and it seized me. "I have him!" cried the voice. "How did you achieve the secrets of invisibility, slave?"

With my left hand I groped for the hilt of the fellow's sword; and when I found it, I said, "You have made a mistake," and drove his sword through the heart of the voice.

There was a single piercing scream, and I was free. Holding my sword point breast high, I turned and ran for the tree by which Ptor Fak had mounted to the roof. One of my shoulders brushed a body, but I reached the tree in safety.

As I climbed carefully to a lower branch so as not to reveal my presence by the shaking of the foliage, I heard a low whistle. It was Rojas.

"Who whistled?" demanded a voice somewhere in the courtyard. There was no reply.

Rojas could not have come at a worse time; I did not answer her; I did not know what to do, but Ptor Fak evidently thought that he did, for he answered the whistle. He must have thought that it was I who was signalling to him.

"They're on the roof!" cried a voice. "Quick! up that tree!"

Now the only tree that overhung the roof was the one that I was in, and if I remained there I was sure to be discovered. There was only one thing for me to do and that was to go up on the roof myself, and I did so as quickly as I could.

I hadn't taken half a dozen steps after I arrived, before I bumped into someone.

"Zodanga?" I whispered. I didn't wish to speak Ptor Fak's name, but I knew that he would understand if I spoke the name of the country from which he came.

"Yes," he replied.

"Find the flier and stay near it until I come." He pressed my arm to show that he understood, and was gone.

I could see the tree up which I had come shaking violently; so I knew that a number of warriors were climbing up in pursuit of me, though how in the world they expected to find me, I don't know.

It was a most amazing situation; there must have been at least a dozen men on the roof and possibly still others down in the courtyard where I knew Rojas to be, yet both the roof and the courtyard were apparently deserted-neither the eye nor the ear could perceive any living thing; only when someone spoke was the illusion dispelled, and presently I heard a voice a short distance away. "He has probably gone this way- the city wall lies nearest in this direction. Spread out and comb the roof right to the city wall."

"It's a waste of time," said another voice. "If someone has given him the secret of invisibility, we can never find him."

"I do not think it was he, anyway," said a third voice; "there is no way in which he could have become invisible-it was unquestionably the ghost of Motus that spoke."

By this time the voices were dwindling in the distance, and I felt that it was safe to assume that all the warriors had gone in search of me; so I walked to the edge of the roof and jumped down into the courtyard. I stood there a moment concentrating all my mental powers in an endeavor to sense the presence of others near me, as Kandus had said that he was able to do, but I got no reaction. This might mean either that I failed to sense the presence of others or that there was no one there-at least near me; so I took the chance and whistled again. An answer came from the other side of the courtyard; I waited.