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MEASURING the shape of the world in the 18th century was a considerable adventure. Astronomers had to haul equipment to remote corners of the globe to look for its subtle deviations from a perfect sphere. This is history writ large, with a long list of characters, and a background of wars, where good maps could be the key to victory. Danson’s narrative sometimes wanders, but his asides can be priceless, like his description of the first British balloonists to cross the English Channel. To keep aloft they had to discard first ballast, then supplies, and ultimately most of their clothing.…