The Jai Prakash Yantra are a pair of hemispherical bowls faced with marble. Like the Ram Yantra, one complements the other – open spaces in one are replaced by surfaces in the other. Inside the spaces are steps for the observers to stand and each bowl was mounted on lead sheeting to ensure it would remain perfectly stable over the years.
The purpose of the Jai Prakash Yantra was to act as a reflection of the sky above. Cross wires stretched over the center of the bowl hold a metal ring, and every point in the sky can be reflected onto a corresponding point on the bowl through the ring. It is said that the Jai Prakash Yantra were so accurate, they were used to calibrate the other instruments in the Jantar Mantar.