This looks interesting, what does it do?

This looks interesting, what does it do?, originally uploaded by spinneyhead.

Found in amongst some of Dad’s stuff, I’m at a loss for what this could be.

It was made by Brown and Sharpe of Providence, Rhode Island- who were well known for making precision measuring devices- based upon a November 28th, 1922 patent. Similar devices turn up in image searches for Brown and Sharpe micrometers, but none of them say what it is beyond “precision instrument&quot”

Any suggestions?

Update It’s a speed indicator. And here’s a little tutorial on how to use one-