Page:Across the Zodiac (Volume 1).djvu/40

 of these were occupied by accurately polished plane mirrors. The central circle, or metacompass, was divided by three hundred and sixty fine lines, radiating from the centre to the circumference, marking as many different directions, each deviating by one degree of arc from the next. This mirror was to receive through the lens in the roof the image of the star towards which I was steering. While this remained stationary in the centre all was well. When it moved along any one of the lines, the vessel was obviously deviating from her course in the opposite direction; and, to recover the right course, the repellent force must be caused to drive her in the direction in which the image had moved. To accomplish this, a helm was attached to the lower division of the main conductor, by which the latter could be made to move at will in any direction within the limit of its rotation. Controlling this helm was, in the open or steering circle on the right hand, a small knob to be moved exactly parallel to the deviation of the star in the mirror of the metacompass. The left-hand circle, or discometer, was divided by nineteen hundred and twenty concentric circles, equidistant from each other. The outermost, about twice as far from the centre as from the external edge of the mirror, was exactly equal to the Sun's circumference when presenting the largest disc he ever shows to an observer on Earth. Each inner circle corresponded to a diameter reduced by one second. By means of a vernier or eye-piece, the diameter of the Sun could be read off the discometer, and from his diameter my distance could be accurately calculated. On the further side of the machinery was a chamber