During Conrad Martens' voyages, including particularly his time on the Beagle, he compiled four sketchbooks. In later life Martens asked his daughter Rebecca to copy what he thought were the most interesting drawings into other sketchbooks, which he kept. The four originals were given to his former favourite pupil, Mrs Macarthur Onslow, and remained for many years in the possession of her descendants at Camden Park, near Sydney; two of them later passed into the possession of Armando Braun Menendez. Through his cooperation and the generosity of Lady Nora Barlow, a granddaughter of Charles Darwin, these two sketchbooks were deposited in Cambridge University Library in 1977.
These two sketchbooks are numbered "I" and "III" on their covers. The first sketchbook in terms of chronological order is, in fact, Sketchbook III. This contains images sketched by Martens during his passage from England to South America, prior to his association with the Beagle, a period which includes all but one of his sketches of Montevideo. Although in Montevideo from August 1833, it was not until November that Martens actually moved in on board the Beagle. The first sketch made while Martens' was a member of the Beagle crew is MS.Add.7983: 21v--22, just before their departure from Montevideo. Sketchbook III measures 140mm high by 220mm wide. On the original cover, the roman number III appears. On the bottom, at the lower end, it is inscribed: `South America'. The Sketchbook has thirty-one leaves. Of the sixty-two pages, twenty are illustrated in pencil, a double page in sepia, and the remaining twelve illustrated pages are watercolours. Most of the sketches are dated, and the book remained in use until early February 1834.
Sketchbook I measures 150mm high by 240mm wide. It has sixty-four leaves. Martens makes over sixty pencil sketches, using for the most part the right hand page. The sketches begin in April 1834, with the exception of the very first image, which dates from late January 1834. It remained in use after Martens' association with the Beagle had come to an end, during his voyage from South America via a number of Pacific Ocean islands to New Zealand and Australia. The last sketch made while Martens was part of the Beagle crew is MS.Add.7984: 38, one of a set made after their arrival in Valparaiso in August 1834.
The sketches were thus made during Martens' encounters with a number of different topographies, climate zones, and human communities. As a result, their content is, not surprisingly, wide-ranging. Topography is one of the most important recurring subject matters, especially because of its central role in the purposes of the Beagle expedition; Martens submitted many of his sketches to the Beagle's captain, Robert FitzRoy, for approval, especially of the depictions of topography, and FitzRoy initialled these after his scrutiny of them. But there are other principal subject matters, including buildings, plants, ships, and human figures, and to a lesser but still significant extent, animals, human social and economic activities, and water (river and seascapes). These main topics form the basis of the classified index points accompanying the descriptions of the images. Cloud types have also been included in this set of topics; FitzRoy was a meteorologist as well as a navigator, and had an apparent influence on the clarity of Martens' depiction of clouds. With some 100 images relating to so many thousands of kilometers travelled, there is little repetition, though some vistas partly overlap, especially those made during the expedition into the Santa Cruz river valley, and those made in and around Montevideo; indeed, apart from glimpses of the Beagle and her support ship the Adventure, one of the few images to reappear a number of times is that of Montevideo Cathedral.