Back when my old man was on the scene the Spice of Life wasn’t a happening place for jazz. It was, according to him, strictly for geezers in roll-necked sweaters and goatees reading poetry and listening to folk music. Bob Dylan played there a couple of times in the 1960s and so did Mick Jagger. But none of that meant anything to my dad, who always said that rock and roll was all right for those who needed help following a beat.

Up until that lunchtime I’d never so much as been inside the Spice of Life. Before I was a copper it wasn’t the kind of pub I drank in, and after I was a copper it wasn’t the kind of pub I arrested people in.

I’d timed my visit to avoid the lunchtime rush, which meant the crowds milling around the circus were mainly tourists and the inside of the pub was pleasantly cool, dim, and empty, with just a whiff of cleaning products fighting with years of spilled beer. I wanted to get a feel for the place and I decided the most natural way to do that was to stand at the bar and have a beer, but because I was on duty I kept it to a half. Unlike a lot of London pubs the Spice of Life had managed to hang on to its brass-and-polished-wood interior without slipping into kitsch. I stood at the bar to drink my half and as I took my first sip I flashed on horse sweat and the sound of hammers ringing on an anvil, shouting and laughter, a distant woman’s scream and the smell of tobacco—pretty standard for a Central London pub.

The sons of Mūsā ibn Shākir were bright and bold and if they hadn’t been Muslims would have probably gone on to be the patron saints of techno-geeks. They’re famous for their ninth-century Baghdad bestseller, a compendium of ingenious mechanical devices that they imaginatively titled Kitab al-Hiyal—The Book of Ingenious Devices. In it they describe what is possibly the first practical device for measuring differential pressure, and that’s where the problem really starts. In 1593 Galileo Galilei took time off from astronomy and promulgating heresy to invent a thermoscope for measuring heat. In 1833 Carl Friedrich Gauss invented a device to measure the strength of a magnetic field, and in 1908 Hans Geiger made a detector for ionizing radiation. At this very moment astronomers are detecting planets around distant stars by measuring how much their orbits wobble and the clever people at CERN are smashing particles together in the hope that Doctor Who will turn up and tell them to stop. The story of how we measure the physical universe is the history of science itself.