Ne plus ultra: or, a sure guide to youth in arithmetick, [electronic resource] : shewing how they may attain to a perfect knowledge of all the common parts of arithmetick, the single and double Rules of Three, direct and inverse, performed by one or two Operations; Fellowship single and double, with the Rule of Practice. Also concise and ready Ways for casting up Bills of Parcels, useful for Merchants in whole-sale Trade, or for Retailers. Together with an easie Rule for casting up Things sold by the Thousand, or bought by the Thousand, and retailed by the Hundred. Likewise simple Interest, so compendiously done, that all Persons in general may reap Benefit thereby, To which is added the method which bankers use for casting up interest by days. By Charles Douglas.