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James Watt's invention of the first practical steam engine helped move Britain's population from a 90% rural basis to a 90% urban basis.

James Watt (1736-1819)


James Watt is chiefly known for inventing different types of steam engine that helped start the Industrial Revolution. To describe the efficiency of his engines, he coined the term 'horsepower' and devised a rev counter.

Watt's inventions were not confined to engines. Examples of some of his other inventions are:

Steam engines – Separate-condenser engine

Watt's interest in steam engines was stimulated in 1763 when he was asked to repair a working model of a Newcomen engine used for demonstrations in Glasgow University.

Newcomen engines had been around since the early 18th century, and were widely used to pump water from mines. They had a coal-fired boiler, with a cylinder on top of it containing a piston.

When the boiler was heated, steam entered the cylinder forcing the piston upwards. Cold water sprayed into the cylinder then caused the steam to condense to water, forming a vacuum in the cylinder. The pressure of atmospheric air pushed the piston down again.

Because the cylinder constantly needed to be reheated, it used up a lot of coal.

Watt pondered over the problem and suddenly had an idea. If there was a separate condenser for the steam, the cylinder could remain hot. This would make the engine much more efficient.

In partnership with Dr John Roebuck, who had set up the Carron Ironworks near Falkirk, Watt began building his own engine. His first prototype was produced in 1768.

Other steam engines

A Boulton and Watt steam engine

In 1768 Watt had travelled to Birmingham and met Matthew Boulton who was running a successful metal 'manufactory' there.

The two of them went into partnership in 1774, and were soon producing engines for use in mines and other industrial sites all over the country.

Other types of engine were soon also being designed. One notable example was a 'rotary engine'. This was an engine in which rotary movement replaced up-and-down piston movement.

The rotary engine was ideal for power looms, bellows, and other mechanical devices. It replaced animal and water power and mechanised industries like weaving and spinning.

Another of Watt's steam engines was a 'double-acting engine'. This allowed the condensation of steam both above and below the piston, giving almost twice as much power.

Together with the engines themselves, Watt was able to produce adaptations so that each engine was customised to suit its purpose.


To describe the efficiency of his engines, James Watt coined the term 'horsepower'. This allowed the output of steam engines to be measured and compared with the power output of draft horses.

The term 'horsepower' was widely adopted to measure the output of piston engines, turbines, electric motors and other machinery.

Horsepower is defined as 550 foot-pounds per second. In modern terms, one horsepower equals 746 watts.

Rev counter

The rev counter measures the rotation speed of a shaft or disk in a motor or other machine.

The instrument is also known as a 'revolution counter' and a 'tachometer'. By the 1840s it was being used on steam locomotives.

Nowadays tachometers are used on road vehicles and aircraft, and in audio tape recorders.

Flexible water main

Watt's work as a civil engineer was highly respected, and his inventiveness can be seen in this type of engineering work too.

He was already in his 70s when Glasgow Water Work Company asked him how they could provide a more efficient clean water supply to Glasgow.

Their main water supply was on the south side of the river, while the city was largely on the north side.

Watt's design, completed in 1810, was for a flexible water mains pipe constructed in the bed of the Clyde.

Flexible mains for water and sewage are now common.


Watt's drawing of his micrometer

Watt's micrometer, designed between 1770 and 1771, was what we would now call a 'rangefinder'. It was used for measuring distances, and was essential for his canal surveying work.

Adapted from a telescope, with adjustable cross-hairs in the eye-piece, it was particularly useful for measuring distances between hills or across water.

The most common way of measuring such distances in Watt's day was by laying a chain on the ground, and calculating the number of chains needed to cover the distance.

Watt produced varying designs of micrometers, but most remained as models only.

Nowadays we use the word 'micrometer' for an instrument to measure tiny objects. It is a completely different device from Watt's. The usual design looks like a question mark, with jaws that open and close, and a cylindrical turning device which displays the necessary measurements.

Perspective machine

Watt's perspective machine

Artists and architects had been using machines to help them draw in perspective since the 15th century. Around 1766, James Watt devised a portable version.

The device was mounted on three legs, and consisted of a box which opened out to form a flat surface, with an adjustable arm holding an eye-piece.

Jointed parallel rulers mounted beneath the board held both a brass point and a pencil, used to trace and sketch the object to be reproduced. The whole device could all be folded up.

Watt estimated that he made between 50 and 80 perspective machines, which were used all over the world. Devices similar to his design are still used today.

Engine detail Link to barometer photo Machine detail Micrometer detail
Portrait of James Watt