When dealing with physical distance, speakers of modern English use either further or farther. In this passage, we see a persistent tendency to replace farther with further: (1) in the 1874 RLDS edition, (2) in the 1906 LDS large-print edition, and (3) early on, when Oliver Cowdery initially wrote down the word in the printer’s manuscript (which almost immediately he corrected to farther by overwriting the u with an a, but without any change in the level of ink flow).
There is a prescriptive grammar rule (discussed as early as 1906 by Frank Vizetelly) that insists speakers should use farther instead of further when physical distance is involved. This grammatical dictum is apparently based on the presumption that farther must be used in order to make the semantic relationship with far more transparent. Yet this rule (and its rationale) is completely artificial and has no historical basis since farther and further are derived from forth, not far. See the discussion under farther, further in Merriam-Webster’s Dictionary of English Usage and also under farther in the Oxford English Dictionary.
Here in Mosiah 19:23, Oliver Cowdery might have had difficulty reading the difference between a u and an a in the original manuscript (which is no longer extant for the book of Mosiah). Such a mix-up could have easily occurred if Oliver himself was the scribe in 𝓞 for this portion of the text since he frequently confused the letters u and a in the manuscripts. For instance, he apparently wrote Cumorah in the original manuscript as if it were Camorah (thus the 1830 compositor set the spelling for this name as Camorah); see the discussion under Mormon 6:2. In other words, what may have looked like farther in the original manuscript might have actually been further. Elsewhere, the earliest text has three occurrences of further:
For each of these examples, all the (extant) textual sources read further, including the last one (which refers to physical distance).
There is, however, additional support in the earliest textual sources for the possibility of farther—namely, in the compound word farthermost:
Here both 𝓟 and the 1830 edition are firsthand copies of 𝓞; therefore the original manuscript most probably read farthermost. The 1837 edition replaced farthermost with furthermost, which has been retained in both the LDS and RLDS texts. This example of farthermost provides additional support for the possibility that the original manuscript for Mosiah 19:23 read farther. The critical text will therefore accept Oliver Cowdery’s virtually immediate correction of further to farther in 𝓟 for Mosiah 19:23. More generally, the critical text will ignore the artificial grammatical rule that requires the use of farther when physical distance is involved; in each case, we will rely on the earliest textual sources. In other words, variation sometimes occurs in the original text, thus farther in Mosiah 19:23, further in Mosiah 24:23, and farthermost in 3 Nephi 4:23.
Summary: Accept in Mosiah 19:23 Oliver Cowdery’s correction of further to farther; the original text seems to have had examples of both farther and further when referring to physical distance.