Fig. 117 shows in what respect the Transit Circle is an advance upon the transit instrument and the mural circle, for in addition to the transit instrument we have the circle. This is a perspective view of the transit, and the telescope is represented sweeping in the vertical plane or meridian. In addition to the instrument resting with its pivots on the massive piers, we have the circle attached to the side of the telescope. We see at once that by means of this circle we are able to introduce the other co-ordinate of declination. If the clock goes true with the earth—if they both beat in unison and keep time with each other—and further if the clock shows 0_h._ 0_m._ 0_s._ when the first point of Aries passes the centre of the field, that is through the meridian plane, then, if we observe a star at the moment it passes over the meridian, the clock will give its right ascension and the circle its declination, when the latitude of the place is known. The construction of the transit circle will repay a more detailed examination. A system of weights suspended over pulleys (Fig. 118) reduces the weight of the instrument on the pivots, in order that their form shall not be altered by too much friction, and on the right-hand side of one of the piers the eyepieces of the microscopes for reading the circle are shown. This is shown better in section in Fig. 119. One of the solid stone piers is pierced through diagonally, as shown at (m) (m), so that light proceeding from a gas-lamp (q) placed opposite the pivot of the telescope is allowed to fall through the openings, and is condensed by means of the lens (n) on the graduations of the circle of five minutes each, already referred to. By the side of each illuminating hole is another hole (o) (o) through which the reading microscopes, six in number, two of which are shown at (q) (p), having their eye-ends arranged in a circle at the end of the pier, are focussed on to the graduations of the circle. There is also another reading microscope, besides the six just mentioned, of less power for reading the degrees, or larger divisions of the circle. Hence from the side of the pier close to the lamp the observer can read the circle with accuracy, and measure the angle, to which we have alluded, made by the telescope when pointed to any particular star. We have now seen how the circle is illuminated, and now we will inquire further as to the arrangements that are necessary in order to bring this instrument into use. [Illustration: FIG. 119.—Plan of the Greenwich Transit Circle. ]