My brother had (has?) a metronome that was big - about the size of a waterbottle - with a weighted arm. You slid the weight up the arm to correspond to the time/beat you wanted and the arm rocked back and forth like ... well, like the dangly things on a grandfather clock (don't know the name of them), only pointed up and not down.
So, I had this image in my head when reading this thread. Hmmmm, I thought, that's not exactly something you could carry in a jersey pocket!
When all else fails, I did what any reasonable modern person would do....
I googled "metronome."
Ahhhhhh, I see, sez the blind man.
Nifty.