ब्राह्मस्फुटसिद्धान्त (ब्रह्मगुप्त - शून्य, कुट्टक, बीजगणित एवं सम्पूर्ण २१ अध्याय सान्वय सटीक)

334 BRAHMAGUPTA AND ASTRONOMICAL INSTRUMENTS
15. Dinārdha yantra—Midday measure instrument.
16. Arka yantra—Sun-instrument.
17. Akṣa or Palāñśa yantra—Small degree measure arc
instrument.
Salila yantra is used for levelling; since a liquid such as
water seeks its own level, it can be utilised to know whether a
surface has been levelled or not.¹ Bhrama or Śāṇa is used for
drawing circles. Avalambaka or plambline is used for
adjusting vertical line. Karṇa is used in connection with angles
and diagonals. From Salila (no. 10) to the last (no. 17); these
eight are used for adjustments and are basically important.
The dhanuryantra is used for nata and unnata kāla
ghaṭikās.
On the paridhi or the circumference of the cakra-yantra
are indicated the twelve rāśis, ending up to Mīna (XXII. 18).
Brahmagupta has described the yaṣṭi yantra and shown how it
could be used to give time at different parts of the day, and
from its shadow dṛgjyā and other characteristics can be calculat-
ed. This instrument can also be used for ascertaining the solar-
lunar differences, and for fixing up the directions. It can
be used for determining various heights and altitudes.
The karttarī yantra is of the shape of a pair of scissors with
two semi-circular blades, fastended to a string at the centre; at
the centre is fixed a pin or a pole which casts shadows.
Setting up of the Gnomon
Here it would be interesting to describe the
setting of a gnomon, which K.S. Shukla has given in details while
commenting on the Mahābhāskarīya (IV. 1) :
After having tested the level of the ground by means
of water, draw a neat circle with a pair of compasses
(karkaṭa) (At the centre of that circle, set up a vertical
gnomon). The gnomon should be large, cylindrical,
massive, and tested for its perpendicularity by means
of four threads with plumbs (avalmbaka) tied to
them.

1. सलिलेन समं साध्यं भ्रमेण वृत्तमवलम्बकेनोर्ध्वम् ।
   तिर्यक् कर्णेनान्यैः कथितैश्च नव प्रवक्ष्यामि ॥ —BrSpSi XXII. 7.