446 
Patents. 
The following letter of Mr. Jefferson, contains so tnuch matter 
worthy of consideration as to patent rights, that I think it worth 
while to subjoin it, without interfering any way, or giving any 
opinion as to the collateral question of Mr. Oliver Evans’s patents. 
It appears to have been written in consequence of a request made 
to Mr. Jefferson, to give his sentiments on the validity of Mr. 
Evans’s claim. I copy it from Niles’s Register, addenda to the 
5th volume. 
Montkeilo, August IS^A, 1813. 
Sir, 
Yoiir letter of August 3d, asking information on the subject of 
Mr. Oliver Evans’s exclusive right to the Use of what he calls his 
Elevators, Conveyers and Hopperboys, has been duly received. 
My wish to see new inventions encouraged, and old ones brought 
again into useful notice, has made me regret the circumstances 
which have follow‘ed the expiration of his first patent. I did not 
expect the retrospection which has been given to the reviving law i 
for although the second proviso Seemed not So clear as it ought to 
have been, yet it appeared susceptible of a just construction j and 
the retrospective one lieing contrary to natural right, it was under- 
stood to be a rule of law, that where the w^ords of a statute admit 
of two constructions, the one just and the other unjust, the former 
is to be given them. The first proviso takes care of those who 
had lawfully used Evans’ improvements under the first patent ; 
the second was meant for those who had lawfully erected ahd tised 
them after that patent expired, declaring they should not be lia- 
ble to damages therefor.” These words may indeed be restrain- 
ed to Uses already past; but as there is parity of reason for those 
to come, there should be parity of law. Every man should be 
protected in his lawful acts, and be certain that no ex post facto 
law shall punish or endamage him for them. But he is endamaged 
if forbidden to use a machine lawfully erected at considerable ex- 
pense, unless he will pay a neW and unexpected price for it. The 
proviso says, that he who erected and used lawfully shall not be 
liable to pay damages : but if the proviso had been omitted would 
not the law, construed by natural equity, have siaid the same thing ? 
In truth both provisos are useless. And shall useless provisos, 
inserted pro major! cautela, only authorise inferences against jus-^ 
tice ? The sentiment that ex post facto laws are against natural 
rights is so strong in the United States, that few, if any, of the 
statp constitutions have failed to proscribe them. The federal con- 
