So it is the 40th anniversary of the mouse, invented as every computer history site will tell you by Doug Engelbart. Every site except PCW, that is. Without wishing to detract from Engelbart's undoubted acheivement, he was not the first to use a trackball, the main component of a mechanical mouse, to control a cursor. That was invented by Tom Cranston (pictured) and his colleague Fred Longstaff as part of a Canadian Navy project that began in 1949.

It is a fascinating story that also reminds us that the graphical user interface did not start with Apple, nor even Xerox.  It began with the wartime radar screens, or even earlier if you count the oscilloscope of which they were a specialist application.

These were of course analogue devices. Cranston and Longstaff were part of a team to computerise the Canadian Navy radar system and they were faced with the problem of placing a cursor over a target blip and registering the corordinates. The simplest way would have been to use two variable resistances, but they came up with a better idea.

Cranston, while on a visit to a naval establishment, had seen someone using a wheel on a stick, like a miniature pedometer, to measure distances on a chart. "We need something like that which works simultaneously in two dimensions," he said to Longstaff.

Longstaff then came up with the idea of two follower wheels resting at right angles to a ball that was free to roll in any direction. The prototype actually used two pairs of wheels driven by a standard 4in Canadian bowling ball resting on an air bearing.

I spoke to Cranston in 2001, when he was 81, and living in Scotland (though he was Canadian), and it felt rather like talking to the man who first stuck a stick in clay to create writing. He and Longstaff, who he believed had died a few years earlier, do not seem to have been aware that they were doing anything momentous: they were engineers addressing a problem.

Cranston was most impressed by the fact that he could address screen positions using numbers, which was novel to anyone used to analogue systems. You can read my full 2001 interview here..

Incidentally, Logitech has just announced that it has shipped its billionth mouse.

PS (updated): I have learned that sadly Tom Cranston died I couple of years after I spoke to him.