Ada Lovelace's Notes on the Analytical Engine

or

Sketch of The Analytical Engine Invented by Charles Babbage; By L. F. MENABREA of Turin, Officer of the Military Engineers; from the Bibliothèque Universelle de Genève, October, 1842, No. 82; With notes upon the Memoir by the Translator ADA AUGUSTA, COUNTESS OF LOVELACE

https://www.fourmilab.ch/babbage/sketch.html

Lovelace's notes start about one third of the way down the page.