416 To manage most grave affairs is bent,

bent > inclined, directed

417 And on whose mighty shoulders most does rest 418 The burden of this kingdom's government

burden > burden; fate, destiny

419 (As the wide compass of the firmament 420 On Atlas' mighty shoulders is upstayed);

Atlas > (A mythical giant said to support on his shoulders the pillars
of the universe)
upstayed > supported

421 Unfitly I these idle rhymes present,

idle > empty; vain, trifling

422 The labour of lost time and wit unstayed:

wit > mind, intelligence unstayed > unsteady