One of his principal achievements in algebra was his demonstration of how to solve quadratic equations by completing the square, for which he provided geometric justifications.[70]:14 Thabit numbers: Named after Thabit ibn Qurra Throttling valve: It appears for the first time in the Banu Musa's Book of Ingenious Devices.[71] Variable structure control: Two-step level controls for fluids, a form of discontinuous variable structure controls, was developed by the Banu Musa brothers.[72] Wind-powered gristmill: The first wind-powered gristmills were built in the 9th and 10th centuries in what are now Afghanistan, Pakistan and Iran.[67] Windpump: Windpumps were used to pump water since at least the 9th century in what is now Afghanistan, Iran and Pakistan.[73]