IV. A second letter from Mr. Colin McLaurin, Professor of Mathematicks in the University of Edinburgh and F. R. S. to Martin Folkes, Esq; concerning the roots of equations, with the demonstration of other rules in algebra; being the continuation of the letter published in the Philosophical Transactions, N° 394

Sir, in the year 1725, I wrote to you that I had a method of demonstrating Sir Isaac Newton's rule concerning the impossible roots of equations, deduced from this obvious principle, that the squares of the differences of real quantities must always be positive; and some time after, i sent you the first principle of that method, which were published in the Philosophical Transactions for the month of May, 1726.