The title of Article 2 is: ``` यनतरबजन — The Four 'Seeds' of Machines: Bhoja's Theory of Energy Sources and Working Media ```

After defining a machine as the constraint of naturally active elements (31.3–4), Bhoja asks where a machine's motion comes from. His answer is the doctrine of bīja, the "seed" of a machine. This article assembles the verses of Samarāṅgaṇasūtradhāra 31 that set out this doctrine: the fourfold bīja of water, fire, earth and air (31.84); the principle that each element is the seed of machines of its own kind and that the elements combine without limit (31.9, 31.16, 31.21, 31.37); the means by which air and fire are captured and held (31.31–32, 31.36); the rejection of mercury as a separate fifth bīja (31.6); and the closing statement that only the bījas, not the constructions, have been revealed (31.80). Read in modern terms, the bīja is the combination of a prime mover and a working medium, and Bhoja's four classes correspond closely to the gravitational-mechanical, hydraulic, thermal and pneumatic sources of modern engineering. The article also explains why space (ākāśa) is absent from the list, and marks the passages whose transmitted text is uncertain. Series: Yantravidhāna Studies, SSD-31/02.