British mathematician whose research and subsequent writing added greatly to the general knowledge of the time on elliptic integrals, calculus, and rotary motion. His studies also included astronomy and physics. He wrote and published a two-volume set called Mathematical Memoirs (1780-89) in which the still notable Landen's theorem appeared. He also tried to simplify the study of calculus by applying it to the accepted principles used in geometry and algebra in his era. Landen was elected to the Royal Society of London in 1766 and lived another 24 years until he died in Northamptonshire in 1790.