Watt conducted experiments in chemistry, particularly on the composition of water. He independently reached the conclusion that water was a compound of oxygen and hydrogen. While Cavendish is commonly credited with 'discovering' hydrogen, Watt was one of the first to propose that water was not an element.

Before his steam engine became a commercial success, Watt worked as a surveyor, involved in projects such as the Forth and Clyde Canal. His surveying skills were valuable in planning and constructing these important infrastructure projects.

Watt also designed and patented a process for copying documents, known as the 'Watt copying machine'. This invention was commercially successful and provided a practical solution for businesses needing to duplicate documents. He created this machine based on observing how his quill pen left a residue that could be transferred.

Watt was a prolific inventor, and developed a technique for bleaching linen, improvements in producing alkali, and contributed to the design of bridges, docks, and harbors. He created a method for measuring distances using a telescope and measuring engine power using an indicator diagram.