Conversation so far:
What was James Watt's first engine?
The Watt steam engine, was one of the driving forces of the industrial revolution. James Watt developed the design sporadically from 1763 to 1775 with support from Matthew Boulton.
When did James Watt make his first engine?
In 1775, James Watt designed two large engines: one for the Bloomfield Colliery at Tipton, completed in March 1776, and one for John Wilkinson's ironworks at Willey, Shropshire.
Did any other models follow the 1775 steam machine designed by James Watt?
James Watt made a number of improvements and modifications to the steam engine. A double acting engine, in which the steam acted alternately on both the piston sides was one

Question: What modifications were made?
Self-contained rewrite: What modifications were made to the steam engine?