His sons took over the workshop and founded James Simpson & Co., which became Worthington Pump Co. through a merger in 1903, renamed Worthington-Simpson in 1917.

At first a small company, it supplied water to parts of Southwark and Vauxhall from pumping works on the south side of the River Thames.

[1] Simpson set up a workshop for repairing and maintaining the recently invented steam engines used by his company.

The workshop would be taken over by his sons and developed into a large steam engine and pump manufacturing business.

At first the Bootle works had just one 2 hp steam engine forcing the water through wooden tubes.

[4] In 1828 James Simpson built the first slow sand filter bed in London for the Chelsea waterworks.

[9] In the 1830s his brother William (1809-1864), the sixth son of Thomas Simpson, ran the engine-making factory in Pimlico.

[10] The same year money was invested to drain and work the Tywarnhayle mine near Porthtowan, Cornwall.

Water was piped out by Cornwall's first electrical centrifugal pumps, made by Worthington, at the rate of 1,000 imperial gallons (4,500 L; 1,200 US gal) per minute.

In 1910 the company's registered office was 153 Queen Victoria St, London, while the works were at Newark.

[20] Worthington-Simpson built a horizontal engine with gear drive for Eastbury Station of the Watford Waterworks in 1920.

[18] The company designed and built two triple expansion steam engines for the Kempton Park Water Works.

[21] At the Mill Dam site near Cottingham, East Riding of Yorkshire, three sets of triple expansion steam engines and pumps from Worthington-Simpson were installed between 1932 and 1934 to extract water from three boreholes to supply Kingston upon Hull.

At the same time, Worthington-Simpson and Worthington Pump and Machinery established agreements to maintain their connections.

[23] During World War II (1939-1945) a Worthington Simpson Pump driven by a Rolls-Royce Kestrel engine was used in a flamethrower known as the Heavy Pump Unit that could project liquid at 3,400 litres (750 imp gal; 900 US gal) per minute, producing a huge jet of flame.

[18] The debt taken on by Weir to acquire their share of Worthington-Simpson was denominated in Deutsche Marks, and as that currency strengthened against sterling and the dollar it became increasingly expensive to service.