mean any number of miles, or any lineal measure. If I apply the hinge C of a pair of compasses to my
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Figs. 10 & 11.
eye, and direct one arm CD to the star A, and the other arm CE to the star B, and then if I observe the inclinations of these arms; if the arms are square, one arm has made one fourth of the complete turn round from the other; and as we call the whole circle 360 degrees, one fourth of the turn round when the compasses are square is 90 degrees, and we say that the stars are 90 degrees apart. If instead of that, I have to put the arms of the compasses in a less inclined position, as in Figure 11, the distance of the stars may be 50 degrees, or 30 degrees, or some smaller number of degrees. This must be fully understood before we can enter upon the explanation of the Mural Circle.
This Mural Circle is an instrument very much varied in form. Figure 12 represents the instruments in use at the Royal Observatory, Greenwich, at Edinburgh, and Cambridge. A is a stone pier which supports the axis of the instrument, and to which
