Muddle 1 of the mint sprigs with the limes, lime juice, and Simple Syrup in the bottom of a bar glass. Add the gin and shake well. Pour into a goblet over crushed ice and stir until the outside of the glass frosts. Top with soda water and garnish with the remaining sprig of mint.
This was the house drink of the Famous 21' Club for years — a kind of Mint Julep for the New England crowd.