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James Rennell

James Rennell - the father of oceanography

The James Rennell Division for Ocean Circulation and Climate is named after Major James Rennell FRS who conducted pioneering work 200 years ago on ocean circulation.

James Rennell "was the founder of oceanography: that branch of geographical science which deals with the ocean, its winds and currents". Thus wrote Clements Markham CB FRS and president of the Royal Geographical Society, in his biography of Rennell in 1895.

Rennell was born in 1742 in Chudleigh, near Exeter, and showed his skill as a cartographer while still at school. His school boy map of Chudleigh is in the archives of the Royal Geographic Society. Soon after his 14th birthday , he joined the Navy as a midshipman, and for the next seven years he sailed the world, saw active service, and learnt his trade as a surveyor, mapping coasts and harbours even in the midst of battle. So strong was his talent that he was appointed, soon after his discharge from the navy, as Surveyor-General of Bengal. The year was 1764, and Rennell was only 21 years old!

During the next 13 years, under Lord Clive and other Governors, Rennell surveyed and mapped Bengal, rising to the rank of Major by 1776. The period was not without incident. His career was nearly terminated in 1766 by a severe sabre-cut in a fight with a fanatical tribe. He recovered, but his health was permanently impaired. In 1772 he married Jane Thackeray (aunt of the great novelist), with whom he found happiness for nearly 40 years until her death in 1810.

When the Bengal survey was completed, Rennell and his wife sailed home from Calcutta in March 1777, arriving in Portsmouth in February 1778. During the long voyage round the Cape of Good Hope, he mapped "the banks and currents at the Lagullas", and his daughter Jane was born in October 1777 during a stopover at St Helena. Rennell's memoir on the Agulhas current (as it is now known) was published in 1778, and was the "very first contribution to the science of oceanography", writes Markham, so that James Rennell "was the father of oceanography".

During the next 50 years, until his death in 1830, Rennell was the leading geographer in England, if not in Europe. He was elected a Fellow of the Royal Society in 1781, and became a great friend of its President, Sir Joseph Banks. He received the prestigious Copley medal in 1791 for his work on India. He was offered the post of first Hydrographer of the Admiralty in 1795, but declined it to continue his academic research, and the post was accepted by Alexander Dalrymple, another great friend and colleague of Rennell.

Rennell returned to his hydrographic work in 1810 at the age of 68, and it occupied him for the last twenty years of his life. He was before all things a sailor, and his numerous naval friends furnished him with a great mass of data from their logs and notebooks, which Rennell sifted and assimilated to chart the currents of the Atlantic Ocean. The work could not have been undertaken earlier, as Rennell himself pointed out, because it was not until the invention of the chronometer in the mid 1700s that longitude could be accurately determined. This accuracy was essential so that current drift could be inferred from the offset between dead reckoned and observed ship's positions. Writing the chapter on the Gulf Stream in 1822, Rennell recognised that "The want of simultaneous observations is an incurable defect". Even today, many oceanographers would agree with him. In an attempt to secure data from sources other than British vessels he asked Sir Joseph Banks in 1819 to approach US sources for data from ships' logs. As no mention is made of such data one concludes that the attempt was unsuccessful.

Rennell looked upon the Gulf Stream in the nature of an immense river descending from a higher level into a plain. The idea was adopted and amplified by Maury in his "Physical Geography of the Sea", but the original idea was Rennell's, and is described in his final work, "Currents of the Atlantic Ocean", published posthumously by his daughter Jane in 1832. He noted the meandering of the Gulf Stream and discussed the formation of large eddies with cold cores - "divides into branches, which have cold water between them". Richardson, writing in 'Oceanography The Past' in 1980, describing the Gulf Stream charts of Frankin and Folger states that "The next two significant improvements in charting the Gulf Stream were by Rennell (1832) and Iselin (1936)". It is surely a tribute to the scientific method of James Rennell that his work was not significantly overtaken for a century.

In addition to Jane, Rennell had two sons, but both died childless. In 1809, Jane married Captain Rodd, later Vice-Admiral Sir John Tremayne Rodd KCB. The Rennell name has continued in the Rodd family. Jane's first son was named James Rennell Rodd, and in 1933 Rennell's direct descendant, a poet and diplomat, took the title Lord Rennell of Rodd when he received his baronetcy. The present incumbent is the third Lord Rennell.

Perhaps James Rennell's greatest service to geography and hydrography was the introduction of scientific method. Rennell carefully examined and sifted all data and theories, and discarded either when unsupported. In consequence, his data were accurate, his methods logical, and his conclusions were right "in almost every instance".

The informal meetings of geographers and scientists that took place regularly in Rennell's and his friends' houses between 1780 and 1830 led to the formation of the Royal Geographical Society two months after Rennell's death. The Society received the patronage of King William IV, to whom Jane Rodd dedicated Rennell's final book. His opening statement, that "the winds are to be regarded as the prime movers of the currents of the ocean" remains unchallenged to this day.

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Last modified: 09:59, 25 February 2009
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