 This did not matter unduly at a colliery, where unsaleable small coal (slack) was available, but significantly increased the mining costs where coal was not readily available, as in Cornwall.Newcomen's engine was gradually replaced after 1775 in areas where coal was expensive (especially in Cornwall) by an improved design, invented by James Watt, in which the steam was condensed in a separate condenser.The Watt steam engine, aided by better engineering techniques including Wilkinson's boring machine, was much more fuel efficient, enabling Watt and his partner Matthew Boulton to collect substantial royalties based on the fuel saved.