It looks like I have yet to respond to your messeage. My gg grandfather was Houton Davy and g grandfather was William E Davy who was born in Iowa in 1879. He lived most of his life in St Joe Mo. All through my life I heard rumors that they were related to Humphry, but I never found out how.
We do have some Davy family items from the early 1800's, but there is a possabliity they are not connected with Humphrey. If Houton is related to Humphry, he would be a grandson or great grandson of a brother of Humphry. I am aware of only one brother, Dr John Davy.