Source is here (Library Of Congress)
Monticello Apr 27. 22
Your letter of the 15th is received, but age has long since obliged me to withdraw my mind from speculations of the difficulty of those of your letter. That there are means of artificial buoyancy by which man may be supported in the air the balloon has proved, and that means of directing it may be discovered is against no law of nature also is therefore possible as in the case of birds. But to do this by mechanical means alone in a medium so rare and unresisting as air must have the aid of some principle not yet generally known. However I can really give no opinion understandingly on the subject and with more good will than confidence wish you succeed.