Samrāṭ-yantra is the famous instrument that uses two garduated arcs separated by a śaṅku (gnomon) wall that casts a shadow on them. If they are large enough, the wall typically has a staircase built right into it.
Bapudeva gives a description of the two samrāṭ-yantra at Varanasi in his work Mānamandiṛa. He described the gnomon wall as being inclined towards polaris, the polar star. He gave the radius of the graduated arcs as 6 cubits, with marks down to 3 aṅgula. He also described the śaṅkupāli (edges of shadow-casting wall) having iron rings coinciding with the cāpapālis (edges of the arcs). 
He described the instrument as being able to show the hour angles of the sun, but also of the planets and moon by way of a metal tube being lined up properly with the aforementioned iron rings and arc edges.  He also describes finding right ascention.
- This tube is reminiscent of the use of the instrument called the nalaka in other contexts.
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