GO 



ANIMAL MECHANISM. 



Of the forms of mechanical work, — When we have at our 

 disposal a certain quantity of force, it is necessary, in order to* 

 utilize it, to collect it under conditions which vary according 

 to the nature of the effects which we desire to produce. 



We have seen that the measure of work actually employed 

 is the product of the resistance multiplied by the space 

 through which it has to pass. Such a measure, being the 

 product of two factors, may remain constant if the two factors 

 vary inversely. So that a considerable weight, raised to a 

 slight height, will give the same result of work as a light 

 weight raised to a greater height. 



These will be two different forms of the same quantity of 

 work ; but, in this case, the form is of extreme importance. 

 In order that the work applied should be available, it is ne- 

 cessary that its form should be the same as that of the 

 resisting force — that is, of the work required to he done. 



If we have as a moving power a piston of a steam engine 

 of large diameter and short length, capable of lifting 100 

 kilogrammes to the height of a centimetre, and that it is 

 necessary with this generator of force to lift one kilogramme 

 to the height of a metre, which equally represents a kilo- 

 grammetre of work, the motive force in this machine cannot 

 be utilized directly ; for at the end of the stroke of the piston 

 the weight of a kilogramme will only have been lifted 

 one centimetre, and -j^^^ of the force at our disposal will re- 

 main unemployed. Every machine, therefore, must be con- 

 structed with a view to the special form under which the 

 resistance to be overcome presents itself. 



It is true that by means of certain contrivances, levers or 

 wheel- work properly combined, it is possible to cause a cer- 

 tain quantity of work to pass from one form to another, and 

 to apply it to the resistance to be overcome. But this will 

 be the object of ulterior study. We have only to consider at 

 this moment the case in which the force is directly applied 

 to the obstacle which it has to surmount, which is a very 

 frequent condition in animated motive powers. 



Let us return, then, to the hypothesis in which the moving 

 force of the piston of an engine must be applied directly to 

 overcome resistance. Under these conditions the constructor 



