Minehead to Orkney, 1807

Here is an entire from Minehead to Kirkwall, Orkney in November 1807.

I required assistance from the internet to understand the postal charge of 1/7d - it appears that either there was an extra 1d charge for the ferry across from Wick to Orkney, or the entire went a long way round (no obvious candidates for the route), or the clerk in Edinburgh got it wrong.