The Sub-Etha Sens-O-Matic is a small black device that alerts hitchhikers to passing spacecraft, by monitoring ships' Sub-Etha signals. Through this device, Ford Prefect learns that the Vogons are on their way to demolish the Earth. Along with his towel, it was one of the permanent contents of Ford's ubiquitous satchel, generally buried under a pile of scripts, to help maintain his cover as an out-of-work actor. It was, of course, light years beyond any Earth technology of the period.