Fill a highball glass with ice.
Add the bourbon.
Top with equal parts cold ginger ale and soda water.
Stir once, gently.
Express a lemon twist over the surface and drop it in.
A pre-Prohibition American highball whose name is a joke about temperate Presbyterians ordering something that looked innocent. Blended Scotch or Canadian whisky works as well as bourbon; the split mixer is the whole idea.
My Bar Shelf reads your shelf and shows you every cocktail you can make from what you already own, including Presbyterian.