The importance of recording climatic data whilst at sea was not the result of purely scientific interest but was a consequence of the need to know the location of the vessel in question. Until the mid-nineteenth century navigation was an imprecise science. The various methods of dead reckoning that were widely employed required information on wind direction and wind force in particular in order to determine the vessel’s daily progress and situation. Disasters as a result of miscalculation were all too commonplace and, importantly for this project, encouraged deck observers to be as diligent as possible in their observations and note-making. Most log books, including the majority from early in the period 1750-1790, recorded the vessel’s speed and the winds every two hours. The cumulative effect of the previous day’s winds were calculated at the start of each nautical day, which was at midday. Log books also registered other aspects of the weather and precipitation, the state of the sea and sky, thunder, lightning etc. were all noted with, seemingly, remarkable consistency. The data for this first half of the study period is, thus, recorded at the finest of temporal scales.

Even after the invention by John Harrison of his famous chronometer and of the publications of various nautical almanacs by which astronomical methods could be used to determine longitude, the habit, sometimes the need, for weather information continued. Log books became a little simpler in layout, noting conditions not every two hours but at each of the three watches into which the nautical day was divided.

Climatologists are fortunate not only in the number of log books that have survived but in the great geographic range that they cover. The use of log books from the British, Dutch, French, Spanish and Argentinean archives allows the data to span the North and South Atlantic Oceans, where all the nations had interests and the Pacific Ocean, which was regarded as a Spanish ‘lake’. British and French interests were focused on links with the colonies in the West Indies and North America and the Indian sub-continent. The Dutch theatre of activity overlapped with those of both the Spanish and British but concentrated on South Africa and the Indian Ocean routes to the East Indies and Japan. Only the most inhospitable extreme northern and southern latitudes lack comparable coverage. But even in these cases the log books from voyages of discovery and occasional trading enterprises shed an illuminating, if discontinuous, light on those areas. The data coverage is thus not only widespread, arguably global, but of sufficient quantity to provide weather data and information from several locations in each of the oceans for each of the days in the century-long record.

Logbook preservation and Study

In Britain the principal sources are the National Maritime Museum (NMM) and the Public Records Office (PRO), both in London, where the logs prepared by masters and captains of Royal Navy vessels are held. To this important source should be added the log books of vessels owned by the British East India Company. These are held in the British Library, also in London. The PRO also holds the log books from the voyages of discovery from this period. Dutch records derive from the Royal Dutch Navy vessels and those of the Dutch East Indies Company. Many are held in the Hague archives and additionally in Jakarta and Cape Town. French sources are concentrated in Service Historique de la Marine and Centre des Archives d’Outre Mer as well as in the Archives Nationales. The principal archives are, for Spanish sources, the Archivo General de Indias (Seville) and the Archivo Museo Naval (Madrid). Closely related to these sources are those held in Buenos Aires from the Spanish colonial period. Many of these log books are derived from the imperial postal system linking Spain to her American colonies which was established under the reign of Charles III.

The log books from the different sources present the observations in a broadly similar manner, reflecting the need to resolve the same difficulties of navigation and administration of vessels at sea with no communications to land for perhaps several weeks, even months. This broad correspondence renders the task of data abstraction less problematical than would otherwise be the case. They differ only in their detailed layout and overall administration. For example, the log books of the Spanish postal service are based on individual return voyages between Spanish and Latin American ports. The log books of both the British East India and the Dutch East Indies Companies are also based on specific voyages from their European bases to some of the furthermost outposts of the two empires. In contrast the log books of the British Royal Navy differ in being linked primarily to the officer and not the voyage. Each officer was obliged to return his log book to the Admiralty upon completion of what would today be regarded as his ‘tour of duty’.

This project concentrates on British, Dutch, French, Spanish and Argentinean sources. The geographical range and volume of log books thereby available are more than sufficient to meet the project’s objectives in terms of geographic and temporal coverage. It is however recognised that other sources exist elsewhere in Europe, some of which are known, others of which are yet to be fully catalogued or even perhaps discovered. In Portugal there have been significant losses of material through fires and natural disasters and the dispersal of much of the remnants to public and private collections throughout the country. The project’s objectives are designed not only to develop the huge volume of data and information in the project’s immediate source material but also to disseminate the findings and encourage others to explore this important source in their own countries, thereby adding further to the knowledge base that this project will provide in the future.