In the first Whiskey vs Whisky article I mentioned that there are a few explanations for why whisk(e)y has two spellings. Masters of Malt claims that in the 1870s American and Irish Whiskey distilleries adopted the spelling with the e as a means to differentiate themselves from the lower quality whisky coming out of Scotland at that time. This seems like a simple statement that should be easy to test, but like many things, the details are significantly more complicated.
My first thought was to look at the etymology. According to Douglas Harper of the Online Etymology Dictionary the word whisky showed up in the early 1700s as a Gaelic translation of the Latin term aqua vitae or water of life. The Oxford Dictionary of English Etymology, has a short entry that lists both whisky and whiskey but it does not include any years or explain why there are two spellings. The Chambers Dictionary of Etymology, has the longest and most detailed entry about the historical development of the word whisk(e)y. It says that from 1583 to 1730 there were at least seven different anglicized versions of the Gaelic word uisge beatha “water of life,” that referred to grain spirits. In 1746 whisky made its first appearance in writing with whiskey following seven years later in 1753. After this no new spellings develop which is probably due to the fact that the first dictionaries were being published around this time. So Chambers offers a simple answer to why whisk(e)y has two spellings; when the word developed it was common to find alternate English spellings for the same thing. However, this does not tell us when each spelling came to be preferred in different English speaking countries.
Chambers and Harper confirm that the spelling of whiskey is most prevalent in the US and Ireland while the spelling without an e is used in England and Scotland. Harper notes that this geographic distinction between the use of whisky in Scotland and whiskey in the US and Ireland was invented in the 19th century. The Ngram of whiskey and whisky used in British English (see the previous post) indicates that after 1850 most British writers used whisky while others continued using whiskey. Even though the Ngrams cannot corroborate the geographic nature of each spelling's use (i.e. Scotland vs Ireland) nor tell us the context in which each spelling occurs, it supports Harper's premise that there was a change in how the spellings were used at that time.
So while whisky and whiskey started off as mid-eighteenth century alternate spellings for grain spirits, by the mid-nineteenth century the two spellings have split in how they are being used. Yet the larger questions of why the alternate spellings of whisk(e)y took on a geographic distinction as Harper and others claims is still unanswered.