Air-conditioning is one of those universal modern conveniences whose origins are entirely unknown to the general public. Online sources credit the first commercial system to the American Willis Carrier in 1902 – but this is not true. The first workable machine was patented four years earlier by Alexander Stewart, a Scottish marine engineer, who called his invention the Thermotank. It offered a massive improvement in comfort for passengers and was rapidly adopted by the shipping industry, eventually equipping many of the greatest liners of their day like _Lusitania_ and _Mauretania_. From these beginnings Alexander and his brothers William and Frederick Stewart built an immensely successful engineering firm with subsidiaries in America, Africa, Australia and Europe. Based on Clydeside, its fortunes were always closely linked to the shipbuilding industry, but with the slump at the end of the First World War the company was forced to look to other markets. At this point Alexander came up with a second world-beating invention, which he called the Punkah Louvre – the swivelling nozzle most familiar today as the source of ventilation in airliners. This made it easier to apply the Thermotank system to other forms of transport and even buildings, greatly expanding its sales potential. Still largely a family concern, the business remained innovative and competitive until the 1960s, when the decline in British shipbuilding and the beginnings of globalisation led to amalgamation, restructuring and eventually the demise of the Thermotank name. In the process, most of the company records were dispersed or lost so this book is a particularly valuable testimony to a great British – specifically, Scottish – success story. Although almost unknown today, Thermotank was a business that changed the world.