4 ideas
From the 1840s the British in India enforced a heavy salt duty with a customs line that eventually ran about 2,500 miles, from the Indus in Punjab to the Mahanadi in Orissa. Under Commissioner Allan Octavian Hume in the late 1860s, its core was a live hedge of thorny plants such as babul, prickly pear and karonda, up to about 12 feet high. It was patrolled by roughly 12,000 men and was abolished in 1879.
Moxham learned the hedge existed from a stray footnote in a colonial official's memoir bought in a London bookshop. He then spent years in archives and on foot in rural India trying to find it.
Salt is a physiological necessity whose consumption the poor cannot cut without harming their health, so a steep salt duty takes a far larger share of their income than of the rich's. Moxham argues the tax made Indians eat less salt than they needed. He holds that this deficiency weakened people's bodies and worsened deaths from heat, disease and famine.
Policing a long internal frontier against smuggling needs vast manpower and invites corruption and harassment of ordinary travellers. The hedge became unnecessary once the government equalized salt duties across regions and collected the tax at the few places where salt was produced or imported. Moving collection to a natural bottleneck removed the need to guard thousands of miles of border.