Those in the cabin were amazed to note how effectually the new muffler and the walls of the cabin shut out the sounds of operation.  It was very easy for them to talk back and forth with each other by using a fairly strong pitch of voice, even when the machine was running at a good rate, as it now began to do, for John once more gave the engine more gas, and turned the airplane skyward.  Up, up they shot like a rocket.  The hand on the dial of the altimeter moved along steadily--it reached 2 again, passed to 3, 4, 5, 6; the earth seemed literally to be falling away from them.  All at once, when they were between six and seven thousand feet high, and watching the minute patches of color far below, which represented buildings, houses, hills, and the like, these objects were swept away, and through the glass plates of the cabin floor they could see nothing but a gray vapor below them.  It was also around them.