Autograph Letter regarding Pendulum experiments in S. America

London: Edward Sabine, 1821. 1st Edition. 1st Edition. Signed by Author. Sabine was an astronomer, geophysicist, ornithologist and explorer born in Dublin, he studied at Marlow & the Royal Military Academy at Woolwich, was commissioned in the Royal Artillery, & served until 1877, retiring as a General. He was appointed astronomer on John Ross's expedition to find the Northwest Passage [1919] & on Parry's Arctic expedition [1819-20[. He conducted valuable pendulum experiments to determine the shape of the Earth at Spitzbergen and in tropical Africa [1821-3], & devoted the rest of his life to work on terrestrial magnetism, discovering a relationship between sunspots and magnetic disturbances of the earth. He was elected president of the Royal Society, 1861. Sabine, General Sir Edward [1788-1883]. Autograph Letter regarding Pendulum experiments in S. America. 1 1/2 pages on beige stock folded to 4" 1/2 x 7" in very good condition [tear where the seal was removed].
Addressed to Mr. J. Brande Esq., London House, n. d. (Friday morning) circ 1821-22.

"Dear Mr. Brande / Any day which you will name next week will suit me equally and I will be much pleased at meeting Mr. McDonich. / I expect a letter from Correa this [morning] recommending Para or the S. American coast as the most suitable spot for the Pendulum Orb. His authority is very good. I believe that there is to be a letter from the royal Society to the Admty on the subject, if so it might be reasonable that you [if you have the letter to write] I have Correa's letter. Shall I send to you for such purposes & wheres. / Yours Ever Most Truly / Edward Sabin / Friday Morg."

In 1821, Sabine turned his attention to the science of geodesy, which had already engaged his attention during the first of his Arctic voyages, and in particular the determination of the length of the seconds pendulum. By measuring the length of a seconds pendulum in different latitudes, one can calculate the "oblateness" of the Earth - i.e. the degree to which the "figure of the Earth" departs from perfect sphericity. Attempts to do this had been made in the eighteenth century, but it was not until Sabine's lifetime that precision instruments were available to allow sufficiently accurate measurements to be made. Sabine threw himself into the task with his usual diligence. Between 1821 and 1823 he travelled halfway around the world with his pendulums and carried out innumerable measurements on the intertropical coasts of Africa and the Americas. He also returned to the Arctic, journeying up the eastern coast of Greenland with Captain D Clavering on Parry's old ship the Griper. Observations were made at Little Pendulum Island, in latitude 74° 30', and among the snows of Spitsbergen. Sabine even had an island named in his honour during this expedition. The results of his research were published "The Pendulum and other experiments", in 1825. They represented the most accurate assessment of the figure of the earth that had ever been made.