Jantar Mantar (Yantra - instruments, mantra - formulae) was constrcted in 1724. Maharaja Jai Singh of Jaipur who built this observatory went on to build other observatories in Ujjain , Varanasi and Mathura. Jai Singh had found the existing astronomical instruments too small to take correct measurements and so he built these larger and more accurate instruments.
The name of this noteworthy astronomical observatory, Jantar Mantar means 'instrument for calculation.' There is a colossal Samrat Yantra at the periphery of Jantar Mantar. To the south of Samrat Yantra there is an amazing instrument called Jai Prakash. The Jai Prakash of Jantar Mantar in Delhi has two concave hemispherical structures and used for determining the position of the sun and celestial bodies. Jantar Mantar is built of brick rubble that is plastered in lime. You can see the following instruments inside the Jantar Mantar of Delhi.