"I have resolved to replace the fifth book by the following treatise. I cannot promise that you will find the study of it easy; but it will certainly be much less difficult than the corresponding portion of Euclid's work; and yon will enter upon it with considerable advantage, if you postpone the attempt—as I here recommend—till you have read, as far, at least, as page 220 of the Algebra. It will, indeed, facilitate your progress, and agreeably diversify your mathematical labours, if you commence the elementary algebra upon closing the fourth book of Euclid, and read the PRINCIPLES through, before you begin the following treatise."