Translation of a letter, from Antoni van Leeuwenhoek to the Royal Society Letter dated 1 April 1689 Observations of flying fish; causes of death in fish; consideration of how fish grow; concerning blood circulation in humans and quadrupeds; consideration of hydrostatics in the circulation of blood; structure of the wall of blood vessels; causation of the rhythm of blood propulsion; circulation of blood in a bat; concerning the noctural flight of bats Translation of EL/L2/25 1689