Letter writing played an especially important role in Charles Darwin's scientific career. Like all scientists, he often needed to communicate with his colleagues, but Darwin's needs were unusual. In the first place, he lived for only short periods in centres where conversation with other scientists was possible. During his student days in Edinburgh and Cambridge he enjoyed personal contacts with university professors and other naturalists associated with the university communities; however, from 1831–1836 he sailed on the Beagle voyage. After his return he lived in London, the centre of British science, but only until 1842 when, partly due to ill-health, he withdrew to the rural village of Down in nearby Kent where he spent the rest of his life. Darwin was not totally isolated in Down. From time to time friends and colleagues paid visits, often to consult about scientific questions. Furthermore, Darwin travelled to London more frequently than has sometimes been supposed. In addition to occasional lengthier visits recorded in his Diary, he also made one-day trips, carefully recording his expenses in his account book. The record of his account book shows that during the early years in Down, he travelled to London more than a dozen times a year, though as he grew older the number of trips declined. Even during the early years, he still depended on the mail for much of the time to keep him in touch with fellow researchers.