Letter, from Antoni van Leeuwenhoek to the Royal Society, dated at Delft Signed autograph 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 Figures in red chalk are attached English translation see EL/L2/26 Transcribed and translated in 'Alle de brieven van Antoni van Leeuwenhoek', edited, illustrated and annotated by Committee of Dutch Scientists, vol 8, p 119 Transcribed in JBO/8/265 1 April 1689