Let the circle, H, Z, R, Fig. 121, represent a great circle of the heavens, the meridian in fact, and let the centre of this circle represent the centre of the transit instrument. Now what we want is, not only to be able to measure degrees of arc along this circle, but to determine some starting-point for those degrees. One arrangement is to observe the reflection of the wires in the eyepiece of the transit circle, from the surface of mercury in a vessel which is placed below the telescope, turned with its object-glass downwards; the vessel containing the mercury is out of sight, between the two piers, but in Fig. 118 are seen the two parallel bars, with weights at the ends, carrying it, by which it may be brought into any position for the purpose referred to, so that the light from the wires in the eyepiece may pass through the tube and be reflected back by the mercury (the surface of which is of course perfectly horizontal), up through the tube again to the eyepiece. When the telescope is absolutely in the vertical position the images of the cross wires will be superposed over the cross wires themselves; and then an observation will give the actual reading of the circle when the instrument is pointing at 180° from the zenith; deduct 180° from this reading, and we get the reading when the instrument is pointing at the zenith—the zero required. This should be 0°, and the quantity by which it differs from 0° must be applied to the observed position of stars, so that the distance of a star from the zenith can be at once determined. But this is not all. If we assume for the moment that the observer is at the north pole, the pole star will be exactly over head, and therefore, supposing the pole star to absolutely represent the pole of the heavens, all the observer has to do is simply to take a reading of the pole star on the arc of his circle—call it 0° O´ 0˝—and then use it as another zero to reckon polar distance from, seeing that every particular star or body we observe has so many degrees, minutes, seconds, or tenths of seconds, from the pole star.