Lift on an airfoil at a given angle of attack may be assumed to depend on the camber alone, and could be modelled by the numerical techniques of the period. Drag depends on the thickness of the airfoil and requires an understanding of viscous flow, which was beyond contemporary capabilities.
The lift on an airfoil at a specific angle of attack can be assumed to be solely dependent on the cambered surface, and could be simulated using numerical methods that were available during that time. Drag, however, is influenced by the thickness of the airfoil and requires comprehension of viscous flow phenomena, which were beyond the capabilities of contemporary scientific understanding.