"The masonry instruments that are the "Jantar Mantar", vary in size from a few feet to 90 feet in height, and are Maharaja Jai Singh's chief work. Jai Singh discarded brass instruments, and built massive masonry ones in their place; his reasons appeared to be, but were not altogether, sound. The brass instruments were, he said, faulty, because of their mobility and size. The axes became worn and the instruments untrue; the graduations were too small for fine measurements, etc.
His remedy was to make large, immovable instruments; but he thus stereotyped his designs, and hindered further improvements. The larger and more immobile an instrument is the greater is the difficulty in making alterations and improvements. Jai Singh sacrificed facility for supposed accuracy."

Source:

The Astronomical Observatories of Maharaja Jai Singh- G.R.Kaye (1918) Published By the Director General, ASI, New Delhi 110001, India