वह्नियन्त्राणि — Heat in Machines: The Fire-born Yantras

Fire is the seed that most interests modern readers of Bhoja, because the heat engine made the industrial world. What does chapter 31 of the Samarāṅgaṇasūtradhāra actually say about fire in machines? This article collects the relevant verses. Bhoja names the fire-seeds as heat, kindling, and the "agitation born of contact with water" (31.28cd–29ab); he says that heat becomes a seed in water-machines (31.34) and, when applied to captured air, in air-machines (31.40); he specifies vessels of clay, copper, iron and gold to hold the fire (31.36); and he lists among the marvels of machines "water rising from the midst of fire" (31.67) and a lamp-figure that keeps its lamp filled with oil (31.71–72). The heated mercury of the flying devices (31.97–99) is reserved for Article 10. Read in modern terms, Bhoja identifies the three ways in which heat can drive a mechanism: by boiling water (steam), by expanding air, and by heating a working fluid in a closed vessel. These are the principles of Hero of Alexandria's fire-driven devices and, much later, of the steam engine. But Bhoja describes no continuous heat engine; his fire-machines produce single effects. The article reads the verses closely, sets them beside Hero's thermal devices, and states clearly what the text does and does not claim. Article 7 of the series Yantravidhāna Studies (SSD-31), on chapter 31 of Bhoja's Samarāṅgaṇasūtradhāra. Verses discussed: 31.28–40, 31.67, 31.71–72. Web version: sanskritmanthan.in/a/ssd-31-07-vahni-yantra