At the Fort Stevens Historical Site & Military Museum parking lot ($5 day use fee), walk across the yard in an easterly direction. Your instrument of magical powers should lead you to the Mine Cable Storeroom. But, remember, your GPS is crow oriented. If you’re unfamiliar with the park, you might want to get a copy of the Ft. Stevens Trail Guide to help find your way.
It’s a nano but don’t be fooled by the appearance.
The log is in the vial. Bring your tweezers. Bring your pen.