The genius of Carl Friedrich Gauss (1777–1855) and the novelty of his work (published in Latin, German, and occasionally French) in areas as diverse as number theory, probability and astronomy were already widely acknowledged during his lifetime. But it took another three generations of mathematicians to reveal the true extent of his output as they studied Gauss' extensive unpublished papers and his voluminous correspondence. This posthumous twelve-volume collection of Gauss' complete works, published between 1863 and 1933, marks the culmination of their efforts and provides a fascinating account of one of the great scientific minds of the nineteenth century. Volume 4, published in 1873, contains Gauss' theoretical work on differential geometry and probability, reviews of work by contemporaries including Herschel, and notes relating to geodesic surveys of the Kingdom of Hanover from the 1820s to the 1840s, together with a description of the heliotrope Gauss invented for the surveys.