This is a fascinating story a young Brahmin who followed Euler's algebra without knowing English, later argued that Bhaskara, 700 years before him, had used the idea behind the differential calculus. Lancelot Wilkinson had lent a young Sanskrit scholar a copy of Euler's book. The young man had only just learnt European letters and numerals. Yet, Wilkinson wrote, he could follow every calculation. Many were "complete puzzles to me". His name was Bapu Deva Shastri.... Wilkinson went further. The pupil was "more fit to be my gooroo (Guru) than my shishya (scholar) in all mathematical question." By 1842 Bapu Deva was teaching European geometry at the Government Sanskrit College in Banaras. Then, on 4 May 1858, he signed a 4 page paper that challenged a received account of Indian mathematics. "It appears to be generally believed that the principle of the Differential Calculus was unknown to the ancient Hindu mathematicians. Allow me to correct this impression..." A 12th century astronomer sat at the centre of the argument. Bhaskaracharya. The question was specific. Find a planet's position today and again tomorrow, and you know how far it moved. But its speed did not stay the same through that day. What was its motion at one instant? Bapu Deva pointed to Bhaskara's two terms: rough daily motion, and tatkalika, the instantaneous motion. His paper explained a construction. Instead of following the bending arc, extend the motion along its tangent. Use similar triangles. A cosine appears in the instantaneous change of the sine. He set this beside a differential formula and argued Bhaskara was "fully acquainted with the principle of the Differential Calculus." In London, H.H. Wilson thought it "a matter of more than ordinary interest" and wanted it carefully tested. He asked the mathematician William Spottiswoode for an opinion. The reply came from 12 James Street, Buckingham Gate. 5 May 1859. Spottiswoode did not accept the whole claim. In the method as given, he found no allusion to infinitesimally small intervals of time and space, and nothing saying the method was approximate. But the letter did not stop there. Bhaskara's penetration was "in the highest degree remarkable". The formula and method bore "a strong analogy" to the corresponding process in modern mathematical astronomy. And of Bapu Deva: "We are much indebted to him for calling our attention to so important an element in the old Indian methods of calculation." The Royal Asiatic Society published the exchange in 1860, with a postscript from Spottiswoode putting the old calculation beside its modern counterpart. Bapu Deva's work went on. A volume of 1861 carried his English translation of the Surya Siddhanta, with Wilkinson's Siddhanta Siromani translation revised by him. On 4 July 1864 the Royal Asiatic Society elected him an Honorary Member. The printed list reads "Pandita Bapu Deva Sastri, Benares", beside Isvarachandra Vidyasagara, Bhau Daji and Syed Ahmad Khan. Sources: JASB vol. 27, pp. 213-216 (1858); JRAS vol. 17, pp. 221-222 (1860); S.R. Sarma, "Sanskrit as vehicle for modern science", p. 195 (quoting Wilkinson); RAS list of members, p. 15. Photo 1: Lancelot Wilkinson on young Bapu Deva: "utterly ignorant of English" yet follows every calculation in Euler's Algebra, and is "more fit to be my gooroo than my shishya." Source: S.R. Sarma, "Sanskrit as vehicle for modern science", p. 195, quoting Wilkinson. Photo 2: Bapu Deva's own claim, signed 4 May 1858: Bhaskaracharya "was fully acquainted with the principle of the Differential Calculus." Source: Journal of the Asiatic Society of Bengal, vol. 27, p. 216. Photo 3: Spottiswoode's reply: no allusion to infinitesimal intervals in the method given, but Bhaskara's penetration is "in the highest degree remarkable" and bears "a strong analogy" to modern mathematical astronomy. Source: Journal of the Royal Asiatic Society, vol. 17 (1860), p. 222. Photo 4: Bapu Deva Shastri teaching at Queen's College, Varanasi, c. 1870. British Library, Photo 1000/46(4709), via Wikimedia Commons. Public domain.