History and root of the principle of the conservation of energy . ms of differentcolours and forms. Without experimental knowledge,he will never discover those circumstances which aloneare relevant. As an example of how important experi-ence is in such derivations, I will give Galileos demon-stration of the law of the lever. Galileo borrowed itfrom Stevinus and slightly modified it, and Stevinussomewhat varied Archimedess demonstration. A horizontal prism A B is hung at the ends by twothreads u and v on to a horizontal bar a b, which canbe rotated about its middle c, or is hung up there by ath
History and root of the principle of the conservation of energy . ms of differentcolours and forms. Without experimental knowledge,he will never discover those circumstances which aloneare relevant. As an example of how important experi-ence is in such derivations, I will give Galileos demon-stration of the law of the lever. Galileo borrowed itfrom Stevinus and slightly modified it, and Stevinussomewhat varied Archimedess demonstration. A horizontal prism A B is hung at the ends by twothreads u and v on to a horizontal bar a b, which canbe rotated about its middle c, or is hung up there by athread. Such a system is, as we see at once, in equilib- CONSERVATION OF ENERGY 67 rium. If, now, we divide the prism into two parts oflengths 2m and 271 by a section at E, after we haveattached two new threads p and q at both sides of thesection, -equilibrium still subsists. It will also stillsubsist if we hang the piece A E in its middle by thethread r, and E B by s to a b, and take away p, q, u, then at a distance n from c hangs a prism of weight Fig. 2m E 271 2w, and at a distance m from c hangs a prism of weight2n. Now the practical physicist knows that the tensionof the threads, which alone mediates between theprism and the bar, depends only on the magnitude,and not on the form, of the weight. Therefore we can,again without disturbance of the equilibrium, replacethe pieces of the prism by any other weights 2m and 2n;and this gives the known law of the lever. Now, he who had not had a great deal of experiencein mechanical things certainly could not have carriedout such a demonstration. Yet another example. At A and B are the equaland parallel forces P and — P to act. As is well known,they have no resultant. Let us suppose, for example,that — R is a resultant, then we must also suppose that 68 CONSERVATION OF ENERGY R is one, for it is determined by the same rule as — Rif we turn round the figure through two right , the one and only resultan
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