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Mathematics. 461 ders of others as well as your own, with the correction thereof. Those who make no mistakes are not always the deepest thinkers; so cheer up and " blunder on," as a muckled-missed old Scot used to say. The rest of the Book consists of propositions, i.e., things set before you, either to be done in some way, (e.g., made, drawn, des¬ cribed, inscribed, &c.,) or else to be proved true. If to be done, the proposition is a problem : if to be proved, a theorem. Now, of the things to be done, some are so easy, that they require no contrivance nor directions for their due performance. They are, therefore, taken for granted, or demanded, at the outset. Such problems are called pos¬ tulates, and are three in number. If you only look at them, you will see that it would be far wiser 'to take a plain ruler and compasses (the only instruments allowed), and do them at once, than to ask your big brother or any one else to shew you how. So, also, of facts in Geometry, some are too plainly true to require any proof, and are, therefore, taken for granted, because no attempt to prove them true could make them any plainer. Such theorems are called axioms. They are twelve in number ; and, to be of any use, ought to be very thoroughly understood. Phlogiston. In order to make me understand Oxygen when I was young (now I am old, I require still more help to take'in all the properties of Ozone !) I was told how the ancient chemists had got up a theory of a certain positive principle of levity or lightness, the removal of which actually caused bodies to weigh more than before! Now, however you may laugh, young friends, this was neither more nor less