about ten to twelve
At last , wearied and sorely puzzled , I turned from it , to view the rest of the strange place on to which I had intruded . Hitherto , I had been so engrossed in my scrutiny of the House , that I had given only a cursory glance ` round . Now , as I looked , I began to realize upon what sort of a place I had come . The arena , for so I have termed it , appeared a perfect circle of about ten to twelve miles in diameter , the House , as I have mentioned before , standing in the center . The surface of the place , like to that of the Plain , had a peculiar , misty appearance , that was yet not mist . From a rapid survey , my glance passed quickly upward along the slopes of the circling mountains . How silent they were .