I cut quite a lot. He envisages a navigable passage through the Vale of White Horse, or part of it:

'The Lake is also divided into two unequal portions by the straits of White Horse, where vessels are often weather-bound, and cannot make way against the wind, which sets a current through the narrow channel.'

I don't know whether that is plausible. Geography is not my strongest point.

Here is a link to the full description.