What was the first convertible car?

I do not know the actual answer but my guess is the 1933 Auburn Boat Tail Speedster, I might have it wrong but - hay might aswell of tried

There was a 1931 Buick roadster that was a convertible with a rumble seat. Also a 1918 Buick Touring Car appears to be a convertible.

http://www.hubcapcafe.com/ocs/pages01/buic1801.htm