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Logbook layout
The general layout of logbooks was consistent over many years but differed in detail from one service to another. Royal Navy logbooks until the early 19th century followed the same system, often with a bare minimum of daily information. The logbook of ships operated by the East India Company are, on the other hand, richer sources of information and officers often recorded meteorological observations on an hourly basis. The logbooks of ships in the service of the Hudson's Bay Company were prepared in a similarly detailed fashion. Although many more Royal Navy logbooks have survived, it was only after 1800 that they began to provide such hourly data.

The general format for the earlier, commonly surviving, Royal Navy logbooks followed a two (facing) page format, the entries being set out as illustrated below:

logbook - left hand page logbook - right hand page It is informative, however, to know a little more of the character of information included under these seemingly obvious headings.

Dates and days of the week
Surprisingly this is not as straight forward as you would expect. Before 1752 the year began on 25 March, or Lady Day in the English calendar. Therefore the 28 February 1743 would be 1744 by our current reckoning. Because of a growing, but informal, recognition that 1 January was a preferred New Year's day, some logbooks adopted a 'double dating' format for the critical January to March period using a style such as '1 March 1749/50'. To confuse matters yet further, the English (Julian) calendar was 10 days behind of the European (Gregorian) calendar until September 1752 when the latter was adopted in England.

The mariner's day also began at a different time. Until an Admiralty order of October 1805, the nautical day began and ended at midday and was 12 hours ahead of the civil day. Midday was of particular importance because this was the time when the major observations were recorded for position, course and distance run in the preceding 24 hours. To add to the difficulty of dating, when a ship entered port the civil day took precedence.

The nautical day was divided into three parts:

• The first part consisted of the afternoon watch and the two 'dogwatches’ (12.00 to 20.00)
  
• The middle part consisted of the first and middle watches (20.00 to 04.00)
  
• The latter part was made up of the morning and forenoon watches (04.00 to 12.00)

The notation 'am' and 'pm' was commonly used and can be considered in their usual sense.

Winds
Wind directions were recorded using a 32-point compass, usually when there was a significant change. There might therefore be several such observations each day. The wind direction was always the point of the compass from which the wind was blowing and was recorded with regard to magnetic rather than true (geographic) north. The following page has more about the problems in measuring and recording wind direction.

Course
The course was the direction of travel 'made good' during the previous 24 hours. This was worked out from a line drawn on a chart between the ship’s position at noon and the position on the previous day. During that time the ship may have steered a number of different courses. The notation used was based on a 360º circle but treated in quadrants. Therefore N67E (67º east of north) corresponds to 67º, S29E (29º east of south) corresponds to 151º and N85W translates to 275º.

Distance
This was the distance covered in the previous twenty four hours between the two noon positions. Unless the ship had maintained the same course for twenty four hours, the actual distance sailed, for instance on a zig-zag course, was slightly more than that recorded. In logbooks where hourly data was recorded, the ship's speed in knots and fathoms was carefully noted. This assisted in working out the ship's position by 'dead reckoning' (see longitude below).

Latitude
Latitude was the ship's position either north or south of the equator. It was determined by an observation of the Sun or the Pole Star. By the 18th century the accuracy of the latitudes recorded can be generally depended on.

Longtitude

Longitude was the ship's position either east or west of a fixed point or zero meridian. In the 19th century Greenwich was adopted as the zero meridian, but before that time any place could be used, the most common being the last observed sight of land before setting out onto the deep oceans. For this reason the Lizard or Land's End were often used when leaving England.

Only after the invention of the ship's chronometer by John Harrison and the refinement of the method of 'lunar distances' by the Astronomer Royal, Nevil Maskelyne, in the late 18th century did the estimation of longitude produce reliable results. Before that time various methods of 'dead reckoning’ had to be used, all of which relied heavily on logbook information.

The zero meridian might change several times in a voyage when dead reckoning was employed. The change commonly took place as landfall was made after a long voyage and the ship's position corrected by reference to some known point such as the Cape of Good Hope or one of the Azores islands. The ship would then set off on the next leg of its voyage with longitude now referring to the new meridian. To work out a ship’s longitude we must determine which zero meridian was being used.

Fortunately, there were a few 'favourites' on the most frequently used routes:

• A voyage from Portsmouth to Madras in India, for example, would use either Greenwich or Start Point as its first meridian.
  
• The meridian would then change on sighting Madeira or one of the Canary Islands. The position of one of these islands would then provide a zero meridian until the ship sighted Cape Agulhas (20º E) at the southern tip of Africa.
  
• After this the vessel might use St. Augustine’s in Madagascar and one of the Comoro Islands in the Mozambique Channel. This would then be used until a landfall was made off Ceylon.
  
• A return voyage from India might use for its first zero meridian either Cape Palmiras (near Calcutta), Cape Comorin (southern end of India) or Point de Galle (off Ceylon).
  
• Voyages clearing the East Indies invariably used Java Head, once clear of the Sunda Strait.
  
• They would use Cape Agulhas when crossing the South Atlantic and then St. Helena for most of the final leg of the journey to Europe.

But until the development of reliable methods of calculating longitude the locations even of these reference points were imperfectly known.

Bearings and distance at noon
This column recorded any significant landmark in sight at noon, giving a bearing using the same notation as the course (see above) and the distance, usually in leagues.

(1 league is 3 nautical miles). If no landmark was in sight the bearing and distance to a major landmark was taken from a chart. This could either be the departure point such as Java Head or an expected landfall such as St. Helena.

Remarks

This section of the logbook was used to record shipboard activities and any remarkable incidents. This could be extremely varied and was recorded chronologically. Activities and incidents may include:

• setting and reduction of sail
• sightings of land
• salutes given and received
• crimes committed and punishment given
• court martial
• deaths
• encounters with other ships (a surprisingly rare occurrence)
• battles (even more rare)
• matters to do with victualling

Incidental meteorological information such as squalls, rain snow or fog were also recorded here, as well as the state of the sky and the sea. An additional piece of information found in the logs of longer voyages were amplitude and azimuth observations. These were taken in order to determine the degree to which true and magnetic north differed. More than other sections, this often gives a vivid picture of life at sea in past centuries.

Additional information
Some officers recorded information not commonly found in most logs. Amongst this information might be found the number of men on the sick list and the amount of fresh water remaining. Much of this was not required by the authorities and reflects the diligence and interests of the officers concerned. Some East India Company ships' officers recorded air temperature and barometric pressure. More importantly for the CLIWOC project, some naval officers and several employed by the East India Company recorded air temperatures and pressure each day. Such observations are of great value in climatological studies, but are rare before 1800 and become commonplace only after the middle of the 19th century. Anyone interested in these post-1850 data should consult the website of the Comprehensive Ocean-Atmosphere Data Set (COADS) where many millions of them have been processed and are available from a database.