'useful arts' in that era meant what we'd call 'trades skills' today.
'To promote the progress of science (total human knowledge) and skilled technical artisans.'
Arguably, given the pace of technical innovation, and the clear effects on independent artisans, there shouldn't be patents at all. Copyright should also be re-evaluated, and if it still exists (it's so very easy to copy anything these days), and targeted towards maximum cultural diffusion of expressions of ideas within pop-culture cycles (20 years sounds LONG for such a timescale).
Trade Marks, however, those are consumer protection and product reputation issues and call for registered (pay a fee to the government) marks that renew as long as paid.
