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Source:Echoes:Ch12:18:Martin Harris on the translation process
Martin Harris on translation
Martin Harris describes how he swapped Joseph's seer stone during the Book of Mormon translation process
Stephen D. Ricks,
Martin Harris aided in the translation of the book of Lehi. Edward Stevenson reported about him:
These and other independent witnesses to the translation provide modern researchers with significant information about the Book of Mormon plates. These firsthand witnesses are an indigestible lump in the throats of those who deny that the plates existed and try to explain the experience as an example of "collective hysteria." 
- After continued translation they would become weary, and would go down to the river and exercise by throwing stones out on the river, etc. While so doing on one occasion, Martin found a stone very much resembling the one used for translating, and on resuming their labor of translation, Martin put in [its] place the stone that he had found. He said that the Prophet remained silent, unusually and intently gazing in darkness, no traces of the usual sentences appearing. Much surprised, Joseph exclaimed, "Martin! What is the matter? All is as dark as Egypt!" Martin's countenance betrayed him, and the Prophet asked Martin why he had done so. Martin said, to stop the mouths of fools, who had told him that the Prophet had learned those sentences and was merely repeating them, etc. 
- Edward Stevenson's account of Harris's Sunday morning lecture in Salt Lake City, 4 September 1870, published in the Deseret News of 30 November 1881 and in the LDS Millennial Star 44 (6 February 1882): 86–87. Cited in Stephen D. Ricks, "Converging Paths: Language and Cultural Notes on the Ancient Near Eastern Background of the Book of Mormon," in Echoes and Evidences of the Book of Mormon, edited by Donald W. Parry, Daniel C. Peterson, and John W. Welch (Provo, Utah: FARMS, 2002), Chapter 12, references silently removed—consult original for citations.
- Stephen D. Ricks, "Converging Paths: Language and Cultural Notes on the Ancient Near Eastern Background of the Book of Mormon," in Echoes and Evidences of the Book of Mormon, edited by Donald W. Parry, Daniel C. Peterson, and John W. Welch (Provo, Utah: FARMS, 2002), Chapter 12, references silently removed—consult original for citations.