Put two ounces of cream of tartar, and the juice and parings of two lemons, into a stone jar ; pour on them seven quarts of boiling water, stir and cover close, when cold sweeten with loaf sugar and straining it, bottle and cork it tight. Add in bottling half a pint of spirits of the best kind.— This is a pleasant liquor and considered wholesome."
"The vintner's guide" by William Phipps, 1825, page 126.
This version sounds rather less alcoholic:
"Recipe for the beverage called imperial pop.—Put into an earthen pot two pounds of sugar, two lemons cut into slices, and two ounces of cream of tartar. Add nine quarts of boiling water, mix the materials well, cover the vessel with a stout cloth and let it cool.
When cold, spread two table spoons full of good yeast from beer on a thin slice of bread and put into the vessel, which must be covered as before, and left till the next day. It may then be filtered through a fine cloth, and bottled and corked tight in strong bottles. In the course of three or four days the fermentation will be nearly complete, and the liquor may be drunk."
"American journal of science, Volume 27" , 1835, page 200.
That's it. Just me being amused by a name.