Both Trevithick and Evans invented the high-pressure engine.

Evans appears to have applied the high pressure principle before Trevithick, and it has been said that Trevithick borrowed it from Evans, but Evans himself never said so, and it is more likely that each of these inventors worked it out independently.

Watt introduced his steam to the cylinder at only slightly more than atmospheric pressure and clung tenaciously to the low-pressure theory all his life.

Boulton and Watt, indeed, aroused by Trevithick's experiments in high-pressure engines, sought to have Parliament pass an act forbidding high pressure on the ground that the lives of the public were endangered.

Watt lived long enough, however, to see the high-pressure steam engine come into general favor, not only in America but even in his own conservative country.
Less sudden, less dramatic, than that of the cotton gin, was the entrance of the steam engine on the American industrial stage, but not less momentous.