While you cannot download the full text, Google Books often has a partial preview of the Theory of Horology. You can use this to study specific chapters (e.g., "Gear trains" or "Lever escapement") legally.
(Eric J. Haswell): A comprehensive 1937 work available via the Internet Archive that details mechanical horological systems. theory of horology pdf download
Time is an abstract concept, but measuring it is a precise science. For centuries, horology—the art and science of measuring time—has stood at the intersection of physics, engineering, and artistry. Whether you are a professional watchmaker, a hobbyist restoring vintage pocket watches, or a student of micromechanics, understanding the theory behind the gears is non-negotiable. While you cannot download the full text, Google
If you are looking for shorter articles or modern technical overviews: The History of Watches Haswell): A comprehensive 1937 work available via the
Unfortunately, I don't have direct access to PDF files or specific articles. However, I can guide you on where to find resources on the theory of horology.