It is not clear who this letter was sent to. It is possible that it was Nevil Maskelyne (as Astronomer Royal, editor of the Nautical Almanac and Commissioner of Longitude), or possibly one of his other British publishers or astronomical correspondents.

Administrative / biographical background
Joseph Mendoza y Rios (1761-1816) was a Spanish astronomer and mathematician, well known for his work on navigation and production of astronomical tables for navigation, which were published in Spain, France and Britain. Educated at the Royal College of San Isidro in Madrid, Mendoza y Rios joined the Spanish navy in 1776. He published a two volume treatise on navigation in 1787, and in 1789 he left Spain to visit Paris and, in 1792, London on behalf of a new institute for navigational sciences in Cadiz. In addition to collecting instruments, manuscripts, charts and books for the institute, Mendoza y Rios established relationships with key European scientists and explored the use of mathematical tables as an aid to nautical astronomy. He remained in London until his death in 1816, and was removed from the Spanish navy list in 1800.