Kinthup had shown himself to be a reliable companion and helper. He was already familiar with the Tsangpo’s route from Lhasa to Chetang, and as far as Gyala Sindong. Kinthup could easily have been an explorer in his own right, but he had neither been schooled nor trained to record field observations methodically. As things turned out, it was Kinthup’s phenomenal memory and his accurate description of places that helped early surveyors pin down quite a bit of the Brahmaputra mystery.

Harman teamed him up with a Chinese lama – whose name, for some reason, was never recorded – for whom he was to act as assistant. Kinthup had already worked as a guide for travellers to upper Sikkim. For his daily bread, though, he ran a tailoring shop in Darjeeling.

In 1880, disguised as pilgrims, Kinthup and the lama headed north from Sikkim towards Gyala Sindong. Their instructions were to move beyond this outpost, tracing the Tsangpo’s flow.

To make sure things went right, Harman devised an ingenious plan. The two were to prepare a large number of logs, and place them into niches carved into these logs, the especially marked “tubes” they were carrying. After sending a message to Harman, the logs were to be thrown into the Tsangpo. Floating down the river, once these logs were found by Harman’s men stationed at the point where the Dihang entered Assam, it would establish once and for all the proposition that the Dihang did indeed join the Tsangpo to the Brahmaputra.

The two crossed the Donkia Pass (or Dongkha La) on August 7, 1880, the same pass Hooker had passed, as had Sarat Chandra Das and Lama Ugyen some time ago. They halted for two days at the Cholamo Lake as they waited for transport to Gyantse. Whenever questioned about their destination, Kinthup had a standard reply: the lama with whom he was travelling was going to pay a visit to his sister, and he was accompanying him as a helper.

Once across the border into Tibet, they joined a group of merchants for the seven-day journey to Gyantse. They remained for nearly a week here, while the lama was entertained by old friends visiting from the Sera Monastery, one of the three great monasteries, together with Drepung and Ganden, on the outskirts of Lhasa (as Nain Singh too had noted).

They continued to travel by begging for food and halting at jikkiyops (or resting places) erected by the Tibetan government to protect travellers from inclement weather and wild animals. Sometimes they spent the nights in caves. By end-August they reached Chetang. The lama fell sick here. For his part, Kinthup, as he later recounted, had already begun to feel “badly treated” and bullied by his companion.

Once the lama recovered, they continued, always keeping the river in sight. An unexpected delay occurred in the village of Thun Tsung, where they had intended to stay for only two days. The lama, it seemed, did not have the heart to leave. A hapless Kinthup was forced to remain there for four months before they could finally leave. In this way, in fits and starts, they reached Gyala Sindong in March 1881.

They never left the river all through this, and walking along its banks, soon came to the village of Pemakochung, which had a small monastery. Only two miles (3.5 km) away, as Kinthup later reported, the Tsangpo “fell over a cliff, from a height of about 150 feet”. Kinthup recollected how the falling water, in play with the light, shaped rainbows at the foot of the mountain.

Kinthup and the lama then moved upriver and turned inland to Jongjuk Jong (now called Tongjuk/Tongkyuk), where the Tsangpo has its most northerly point. The bridge leading to Tongjuk was manned by an elderly man who checked for permits issued by the Dzongpen, the head of the district. There was a delay again as the lama and the elderly man left to get the necessary permit from the Dzongpen. Kinthup remained by the outpost fort near the bridge and looked for a chance to hide his three compasses and pistol. The lama returned four days later with the required permit. In the fort, Kinthup boarded with the Dzongpen’s helpers, and they re-equipped themselves with flour, meat and tea needed for the next stage of the journey.

For Kinthup, though, things soon took a turn for the worse. The lama spent far too much time with the Dzongpen, and he worried the lama would soon disclose the real nature of their travel to him. Soon enough, one of the Dzongpen’s helpers came up and threatened Kinthup with dire consequences if he didn’t come clean about what he was hiding away. Kinthup was forced to disclose what he had stowed away – the compasses and his pistol.

Excerpted with permission from Flying Horses, Secret Rivers, Magical Cities: Incredible Adventures in India and Beyond, Anu Kumar, Hachette India.