. "ts Cultivate and Improve three acres Part of every fifty of the Tract abovementioned Then the Estate hereby granted shall Cease & be utterly Determined and thereafter it shall and may be lawful to and for us our Heirs and Successors to grant the same Lands and Premisses with the Appurtenances unto such other Person or Persons as we our Heirs and Successors shall think fit In Witness whereof we hav" . . . .