An Observatory in itself
The scale's centre is located 27 degrees below the edge. The connecting line between the scale's centre and the metal ring points exactly to the Celestial North Pole. Proceeding from that point equatorial coordinates of the celestial bodies can be read. The circle around the centre shows the position of the Ecliptic Pole througout the year. It helps determining celestial coordinates, too. All those observations of the sky are feasible just with the Jai Praksh Jantra. Maybe it is Jai Sing's major work, particularly because it lies in the centre of Jantar Mantar and there is a smaller precedessor west of it.
In the bottom third the centre of the scale and above the
matal ring that holds the tube.
On the left the inside surface of the hemisphere.