MURAL INSTRUMENTS

These instruments, very broadly, consisted of a telescope that could be moved around a single axis along the plane of the meridian, so that only the stars which at that moment reached culmination could be seen. A graduated limb, on which the part of the telescope turned towards the observer moves, measures the altitude of the stars. Transit times are computed by reading the time on a clock at the moment of the celestial objectís transit over the meridian.
The mural quadrant may be considered the predecessor of the more modern Meridian Circle; it consisted of a graduated metal quarter circle fastened to a vertical wall that had to be oriented in a North-South direction. Around a horizontal axis, fixed at the centre of the quadrant, moved a telescope lying in the plane of the meridian which the observer could position as he wished. By using the scale on the limb it was possible to compute the elevation of the observed celestial object on the horizon or else its declination. To cover the whole visible hemisphere the instrument could also be applied to the other side of the same wall so that after looking at the zenith from the southern cardinal point one could then view the opposite quadrant from the northern cardinal point to the zenith. More complete instruments consisted of a mural semicircle which spared the observer the actual job of inverting, even though the wider range of the graduated limb and the bigger size of the instrument inevitably reduced observation accuracy.