The Great Arc – John Keay

I’ve seen many a ‘trig point’ whilst walking the hills of Britain, these mainly concrete structures on top of high points were used for accurate mapping, specifically to get the correct height of hills and mountains, but quite how they were used was not something I particularly thought about before reading this book. The story John Keay tells is of an epic fifty year project to both start the accurate mapping of India but more importantly to create the longest ‘Great Arc of the Meridian’ a accurate calculation of the curvature of the Earth and it’s variation as you move from the equator to the north pole, one of the most outstanding scientific endeavours of the first half of the 19th century. Started in 1800 by a team led by William Lambton and ultimately completed by George Everest (pronounced ‘eve rest’ not ‘ever rest’ as he and his descendants would repeatedly tell people) the sheer scale of the project can be seen on the map below as a series of phenomenally precise triangles stretch all the way from the southern tip to India right up to the foothills of the Himalayas.

The basic concept is quite simple, first establish a baseline whose length is exactly known but is also long enough to mean that a high point visible from both ends will form a significantly different angle when this is measured by a theodolite from these two points. Using trigonometry you can then calculate the position of this third point and the length of the two inferred sides of the triangle formed. One of these ‘new’ sides can then become the base of another triangle, a new high point selected, measured and so on. It had already been established that the Earth wasn’t round like a ball but more like a grapefruit so flatter at the poles than at the equator but by just how much was it flatter. Measurements had been taken of the length of a degree (1/360 of the circumference of the Earth) and it had been found that in Ecuador (on the equator) it was approximately 111km whilst in Lapland it was around 110km so a whole kilometre shorter.

The problem lies in accurate measurement of a long enough distance, nowadays it is relatively easy but over two hundred years ago the equipment was a lot more primitive and Lambton had to use what was called a chain but was a lot more sophisticated than that. His was made up of forty bars of blistered steel each two and a half feet long and each attached to the next one using a brass hinge, using this he had a measure of one hundred feet (30.48 metres) that he knew to be correct, the problem comes when he needed a long enough base to his first triangle which he decided was a seven and a half mile long (12.07 km) flat stretch of land that needed to be cleared and levelled as much as possible near Madras. Which means that he had to use his chain four hundred times, precisely starting where the previous measure had finished, in a perfect straight line and allow for the expansion of the steel as its temperature rose under the Indian sun even though he only took measurements in the early part of the day. It would take fifty seven days to complete the seven and a half miles and the markers for the two end points can still be seen. From this line he could head north.

Now you have probably seen surveyors with theodolites at building sites but nothing like the giant piece of equipment Lambton used. It needed to be this size not only for stability but to allow for the large brass dials which would make the scale large enough to read extremely accurate measurements of the angles and even then the dials were fitted with microscopes so that the precise figure could be attained. Lugging this massive instrument across India, through jungles, deserts, up mountains and all sorts of other terrain never mind crossing rivers along with all the other equipment, food and tented accommodation for the entire vast team for months at a time was a stupendous achievement with people falling ill or dying both of sickness and animal attacks throughout the fifty years of the survey. Each time it was set up it had to be on a high point with other members of the team at another high point with a marker, initially flags and then later on lights and sometimes it would take weeks for the marker team to reach the next point, it was very slow progress with trees and in some cases houses or parts of whole villages having to be cut down or purchased and then flattened to provide clear sight lines from one point to the next. Six years after starting out a new base line was measured to check the calculated length with reality and amazingly over the six miles (9.66 km) checked the error was just 7.6 inches (19.3 cm) or to put it another way he was out by just 0.0000002%.

William Lambton eventually retired and was replaced by George Everest who carried the survey up to the foothills of the Himalayas but not into Nepal as that kingdom was going through one of its reclusive periods and they were not allowed in even to do scientific work. Besides it was known that the theodolite could see vast distances, possibly even into women’s quarters, and even worse the image seen was inverted and no man wanted his wife, or wives, seen upside down so they were often attacked by villagers or blocked by local rulers from coming through certain parts of India. This added to the geographic, animal and disease problems really slowed progress but Everest was not a man to put up with resistance to his survey and he pressed on regardless. He never saw the mountain that was to be named after him when it was determined to be the world’s highest peak; but nowadays whilst everyone has heard of Mount Everest, who has heard of George Everest? Tragically especially ignored is the brilliant William Lambton who started this magnificent survey so this book is important to raise their profile again. It is also a fascinating description of the hardships endured by the teams who did this amazing project. John Keay has produced a highly readable account of the survey which whilst including details as to how the work was done never gets bogged down in the mathematics which is a trap that would have been so easy to fall into. It was first published in 2000, mine is the 2001 paperback published by Harper Collins and is still easily available and I highly recommend it.

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