Peering cautiously through the vistas in various directions, we ascertained our location with respect to the wall of the building. Like all the structures that we had seen on Mars, it was composed of polished red metal. Looking for an Entrance. "Where is the entrance?" inquired Mr. Edison, in a whisper. "Come softly this way, and look out for the sentinel," replied Aina. Gripping our disintegrators firmly, and screwing up our courage, with noiseless steps we followed the girl among the shadows of the trees. We had one very great advantage. The Martians had evidently placed so much confidence in the electric network which surrounded the power house that they never dreamed of enemies being able to penetrate it--at least, without giving warning of their coming. But the hole which we had blown in this network with the disintegrators had been made noiselessly, and Mr. Edison believed, since no enemies had appeared, that our operations had not been betrayed by any automatic signal to watchers inside the building. Consequently, we had every reason to think that we now stood within the line of defence, in which they reposed the greatest confidence, without their having the least suspicion of our presence. Aina assured us that on the occasion of her former visit to the power house there had been but two sentinels on guard at the entrance. At the inner end of a long passage leading to the interior, she said, there were two more. Besides these there were three or four Martian engineers watching the machinery in the interior of the building. A number of air ships were supposed to be on guard around the structure, but possibly their vigilance had been relaxed, because not long ago the Martians had sent an expedition against Ceres which had been so successful that the power of that planet to make an attack upon Mars had for the present been destroyed. Supposing us to have been annihilated in the recent battle among the clouds, they would have no fear or cause for vigilance on our account. The entrance to the great structure was low--at least, when measured by the stature of the Martians. Evidently the intention was that only one person at a time should find room to pass through it. Drawing cautiously near, we discerned the outlines of two gigantic forms, standing in the darkness, one on either side of the door. Colonel Smith whispered to me: The Disintegrator Again. "If you will take the fellow on the right, I will attend to the other one." Adjusting our aim as carefully as was possible in the gloom, Colonel Smith and I simultaneously discharged our disintegrators, sweeping them rapidly up and down in the manner which had become familiar to us when endeavoring to destroy one of the gigantic Martians with a single stroke. And so successful were we that the two sentinels disappeared as if they had been ghosts of the night.