He served in the Army from 1721 to 1747 .
Was aide - de-Camp to his uncle , the John Dalrymple , 2nd Earl of Stair , at the Battle of Dettingen in 1743 .
He served in the Army from 1721 to 1747 , and was aide - de-Camp to his uncle , the John Dalrymple , 2nd Earl of Stair , at the Battle of Dettingen in 1743 .