Crazy fuckers!
I sometimes wonder if it's a once common skill that's just been largely lost and now only really pursued at large these days by thrill seekers.
That Liver building picture must have been c1912 or so but when you look at a picture of the docks from c1890's, here's one of Salthouse...
All those sailing ships needed people able to climb up the masts and walk along them to set the canvas, and in all weathers.
It must have been an unimaginably harsh and perilous existence.