But fortuitously for Watt, the partnership failed when Roebuck's business hit hard times. It allowed him to form his famous partnership with Matthew Boulton at Birmingham's Soho Works in 1774.

Watt's engine was quickly seen to be superior to Newcomen's. It was more powerful, more reliable and used only about a third as much coal as its predecessor. With successive patents between 1781 and 1785 for improvements, such as the sun-and-planet motion, the expansion principle, parallel motion and a smokeless furnace, he further refined the system.

An air pump, a steam jacket for the cylinder and the double-acting principle also flowed from his fertile mind.

Boulton and Watt engines were soon replacing uneconomic Newcomen machines in Cornish mines. Boulton introduced a novel charging system to help sell the engines: The customer paid for the parts and labour, plus an annual percentage of the saving in the cost of coal compared with a Newcomen device.

Having perfected the pumping engine to his own satisfaction, in 1781 Watt turned his attention to developing a rotary motion, rather than the up-and-down motion of the pumping engine.

Within two years, Richard Arkwright had a Watt engine installed in his Nottingham mill, and soon Soho-built engines were powering the cotton mills of Lancashire and were in use in industry throughout the country. By 1800, when his patents finally expired, there were more than 500 of Watt's machines in Britain's mines and factories.

However, it is impossible to escape the conclusion that the dog-in-a-manger attitude of Watt and Boulton delayed the course of the Industrial Revolution for several years. And in the end, their cumbersome, low-pressure engines proved a blind alley. High-pressure, non-condensing engines were the way forward.

But the pair protected their interests so well that virtually all development in that direction was stymied - even William Murdoch, one of their own employees, was barred by law from developing the steam carriage he had invented.

Nothing would induce Watt to develop engines using what he termed "strong steam". He felt the idea was inherently dangerous. Thankfully, others thought differently and with the expiry of Watt's patent, men like Richard Trevithick moved in to take steam power to the next level, with more powerful, lightweight engines that could be mounted on wheels and made to pull wagons. Without high-pressure steam, the age of the locomotive would not have dawned.

Watt retired in 1800, having done much to transform the industrial landscape. He died at his home, Heathfield Hall, near Birmingham, in 1819. The watt, a unit of power, is named after him, and it was he who first coined the term 'horsepower.'

In 1817, Watt's son, James, fitted the engine to the SS Caledonia, the first ocean-going steamer to leave port.