The Hemicycle is not certainly known to have been invented until long after this, about B. C. 350.
— from Time Telling through the Ages by Harry Chase Brearley

The real reason for this habit is not certainly known.
— from The Life and Letters of Ogier Ghiselin de Busbecq, Vol. 1 (of 2) by Ogier Ghislain de Busbecq

In answer to this, I think it is now generally admitted that he is not certainly known earlier than the Post-glacial period.
— from The Chain of Life in Geological Time A Sketch of the Origin and Succession of Animals and Plants by Dawson, John William, Sir

In doing so, you will give me a pleasure the highest I now can know.
— from Clarissa Harlowe; or the history of a young lady — Volume 6 by Samuel Richardson

The truth of this, however, is not certainly known, nor can its success be depended on.
— from The Writings of Thomas Jefferson, Vol. 1 (of 9) Being His Autobiography, Correspondence, Reports, Messages, Addresses, and Other Writings, Official and Private by Thomas Jefferson

Oh! do not stand; see, how my bosom heaves To meet the stroke; in pity let me die, 'Tis all the happiness I now can know.
— from The Prince of Parthia A Tragedy by Thomas Godfrey