The writer of a hitherto unnoticed letter dated 17 January 1770, which was received by the National Meteorological Library and Archive, which was received in 1935 by the National Meteorological Library from the Radcliffe Observatory, Oxford, has been identified as Spencer Cowper, Dean of Durham from 1746 to 1774. In the letter Cowper summarizes the weather observations he made in Durham in 1769, provides a short but masterly analysis of the regional climate and refers to the completion of a naturalist's journal, which he planned to send to Oxford. This paper makes the case that the recipient of the letter was almost certainly Thomas Hornsby, first Radcliffe Observer, whose links with Durham are fully explored. Investigation of the context in which the letter was written reveals the hitherto neglected range and depth of Spencer Cowper's scientific interests and connections. The journal to which Cowper refers has not yet been traced despite extensive searches, but, through an examination of the development and use of naturalists' journals during the late 1760s and of the wider national and regional context of data collection and observation evident in correspondence between those involved, new light is shed on Spencer Cowper's likely awareness and associations, some of which are evident in the newly analysed catalogue of his books and scientific instruments that were offered for sale after his death.