6 mins!! Well done.
Evidently '1st bridge' was a slab of stone across the chasm for pedestrians.
1713 lower bridge built.
1816 replaced with the one we all now use - to reduce the gradients down and up from the 1713 bridge.
Saw this on an old 'Escape to The Country' from Perthshire - they were looking at houses to the North of Perth - so not much help with my Google searches. And it took me more than 6 mins believe me!!