Chains of triangulation, stretching for hundreds of kilometres along the Earth's meridians, were designed as scientific experiments. Their purpose was to reveal the size and shape of the Earth, one of many topics debated by 18th-century scientists sitting comfortably in their studies.
They agreed the Earth was almost spherical, but some (following Newton) said it bulged slightly at the equator and flattened towards the poles.
Others (following the Cassinis) said it was elongated towards the poles and compressed at the equator.
The French Academy of Sciences decided to resolve the dispute. Measurements by triangulation of two arcs of meridian were planned, one in Peru, close to the equator, the other as far north as possible, in Lapland. If a degree of latitude in Peru were found to be shorter than a degree of latitude in Lapland, Newton's view would hold. If the reverse were found, the Cassinis would be justified. If no difference was found, then either the Earth is a perfect sphere or the measurement errors were too great to detect any flattening or elongation. Scientists would have to forsake the comfort of their academies for a few years to settle the dispute.
To write about the expedition to Peru, Robert Whitaker followed the example set by the early scientists. He left his study to seek first-hand evidence of the places where the scientists lived and travelled, and of the history of people who became involved in the difficult enterprise. Whitaker has succeeded at two levels. His descriptions of the scientific work and the excessively harsh conditions under which the most delicate and accurate measurements had to be made are easy to follow, but he has not compromised on the science (Newton turned out to have been right, the Cassinis wrong).
He has also written an enthralling narrative of quarrels among scientists, acts of charity and betrayal, and the love between a wife and husband that survived more than 20 years of separation.
When his work had been completed, Jean Godin (the expedition signal carrier and cousin of Louis Godin, one of the three scientists in charge of the expedition) intended to take his Peruvian wife, Isabel Gramesón, and their young family to Paris. He set off to investigate a route across the Andes, down tributaries of the Amazon and the great river itself to French Guiana, where he became stranded. After 20 years of silence, Isabel set out to follow her husband's intended route to try to discover what had happened to him. Whitaker's account of her journey, her indomitable spirit despite the deaths of her companions, and of the suffering she endured when travelling alone on the Amazon, is a harrowing but inspiring tour de force .
Measurement of arcs of meridian in the 18th and 19th centuries demanded great physical courage. Details of the remaining concrete station marks in the Struve arc, measured from 1816-65 and stretching for almost 3,000km from the Arctic Ocean to the Black Sea, have been submitted for designation as a World Heritage Site. Whitaker's book stands as a monument to people who showed great courage in the pursuit of knowledge of the natural world.
It is still important to measure the shape of the Earth because deformation under the influence of sea-level loading and internal stresses within the mantle can have disastrous consequences.
Michael Cooper is emeritus professor of engineering surveying, City University, London.
The Mapmaker's Wife: A True Tale of Love, Murder and Survival in the Amazon
Author - Robert Whitaker
Publisher - Doubleday
Pages - 352
Price - £16.99
ISBN - 0 385 60520 X