A dissertation concerning the arithmetick of annuities : wherein it is shewn, that this branch of arithmetick is hitherto deficient, with respect to true rules, deduc'd from the only true and just supposition of discounting simple interest, for resolving questions concerning the present worths, rates, times of continuance, &c. of annuities, leases, mortgages &c. at simple interest : to which are subjoin'd proposals for supplying this defect /