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so on, they have furthered a good result less than they believe. For, aa we have seen in the text, there is really some truth at the bottom of these problems no precautions have been able to expel the sense of this fact. We read between the lines the sentence of Horace : Naturam expellas furca, tamen usque recurret !
It is not easy to say up to what limits it may be advisable for general Mechanics to follow the methods which we have worked out. I believe, how- ever, that it is certainly advisable that the simple machines should be treated in the way which our kinematic investigations have pointed out to us in elementary Mechanics. This cannot but be of use in increasing the tangibility and definiteness of the ideas which the scholars receive upon the subject.
To the question how far generally Mechanics should concern itself with machines, we may say that this should be the case in decreasing degree from the lower to the higher Mechanics. For those who want merely elementary notions of the subject, general Mechanics and the Mechanics of machinery are one and the same thing. The higher the studies be pursued the more dis- tinctly their differences make themselves felt. Which of the many positions between the highest and the lowest should be adopted in the construction of a text-book must in every case be carefully considered. But before every- thing in my opinion elementary text-books of Mechanics deserve much more careful treatment, especially as to the logical arrangement of their contents, than they have hitherto received. They are too often deficient in that trans- parent clearness which we are entitled now to demand from Mechanics. We have already noticed this in reference to friction. How unconnected with every- thing else, also, the treatment of the strength of materials commonly is ! Apart from certain internal peculiarities in the way of new data, to which I have called attention in the preface to the last edition of rny Constructeur, the general treatment of the matter appears to me defective, and it is never made sufficiently distinct that the " strength of materials " stands simply in the same relation to rigid bodies as hydrostatics and hydraulics occupy to liquids and aerostatics and aerodynamics to gaseous bodies. (If we wished to include all under a general title we might use the words stereostatics and stereodynamics for that purpose.) All three branches treat of the inner mechanical forces our latent forces of 1 which give the material its existence ; all three besides overlap each other in the limiting cases. On the other hand just the same separation can be made with fluids as is done in the separate treatment of the " strength of materials " with solids ; the problems connected with their molecular condition can be separated from those concerning their relations as a whole to other bodies. Very valuable analogies show themselves between the three depart- ments if they only be looked for. I believe that new life might be thrown into the whole study if its treatment were taken up afresh in the direction which I have pointed out. [I may take this opportunity of pointing out that we have already in English an excellent word, introduced by Prof. Rankine, for for what Prof. Reuleaux calls a "latent force," namely stress. I am not aware that it has any German equivalent. It is very much to be wished that engineers and physicists too, for that matter would agree to use this word where now strain is often employed, and to keep the latter for its more obvious meaning of deformation, for which it is far better adapted.]
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