"If one has been put under duress until he sells, and takes the purchase-money, even if they hang him up till he sells, yet the sale is valid, whether of movables or of lands, and this though the price has not been accepted before witnesses. Hence he should make his protest before two witnesses, and say to them: 'Know ye that I sell this field [or this article] under compulsion.' If the seller does this, the sale may be set aside after many years' possession, and the buyer must make restoration. But the witnesses must know of their own knowledge that force was used; and when the protest is written out to be signed by them, it should recite such knowledge on their part. This refers only to a sale of property or to the compromise of a claim; but a gift of property, or the free release of a claim, is void whenever the donor or releasor protests his unwillingness at the time, though he be not under duress at all. Beating or other bodily violence is not the only form of duress; duress may consist in the threat of any harm which it is in the power of the other party to inflict. . . . But no protest is necessary to prevent the possession of land which is taken by sheer violence from ripening into a title by prescription. An admission made by the seller after the protest does not estop; for it is presumed that he was forced to make it"