Physics

Free to read here, from Project Gutenberg.

Opticks

Isaac Newton, also an audiobook

Relativity : the Special and General Theory

Albert Einstein

Relativity: The Special and General Theory

Albert Einstein

Philosophiae Naturalis Principia Mathematica

Isaac Newton

Latin

Relativity: The Special & the General Theory

Albert Einstein

The Meaning of Relativity

Albert Einstein

Science and hypothesis

Henri Poincaré

The New Physics and Its Evolution

Lucien Poincaré, also an audiobook

The Theory of Spectra and Atomic Constitution: Three Essays

Niels Bohr

The Einstein Theory of Relativity: A Concise Statement

H. A. Lorentz

A Brief History of Element Discovery, Synthesis, and Analysis

Glen W. Watson

The Theory of Heat Radiation

Max Planck

Utility of Quaternions in Physics

Alex. McAulay

LRL Accelerators, The 184-Inch Synchrocyclotron

Lawrence Radiation Laboratory

Expériences et observations sur l'électricité faites à Philadelphie en Amérique

Benjamin Franklin

French

The Machinery of the Universe: Mechanical Conceptions of Physical Phenomena

A. E. Dolbear

Elementary Principles of Statistical Mechanics

J. Willard Gibbs

Space—Time—Matter

Hermann Weyl

Five of Maxwell's Papers

James Clerk Maxwell

On a Dynamical Top, for exhibiting the phenomena of the motion of a system of invariable form about a fixed point, with some suggestions as to the Earth's motion

James Clerk Maxwell

Ancient and Modern Physics

Thomas Edgar Willson

Treatise on Thermodynamics

Max Planck

Space, Time and Gravitation: An Outline of the General Relativity Theory

Arthur Stanley Eddington, Sir

Eight Lectures on Theoretical Physics, Delivered at Columbia University in 1909

Max Planck

The Mathematical Theory of Relativity

Arthur Stanley Eddington, Sir

On the Quantum Theory of Line-Spectra, Part 1 and 2

Niels Bohr

The Theory of the Relativity of Motion

Richard C. Tolman

Vorlesungen über Thermodynamik

Max Planck

German

Notes on Recent Researches in Electricity and Magnetism

J. J. Thomson

Matter, Ether, and Motion: The Factors and Relations of Physical Science

A. E. Dolbear

A Fortran Program for Elastic Scattering Analyses with the Nuclear Optical Model

Michel A. Melkanoff and others

Moderne Probleme der Physik

Hermann Sieveking

German

Die Potentialfunction und das Potentiall; ein Beitrag zur mathematischen Physik

R. Clausius

German

Sechs Vorträge über ausgewählte Gegenstände aus der reinen Mathematik und mathematischen Physik

Henri Poincaré

German, French