BY R. H. J. BROWN
Department of Zoology, University of Cambridge
This contribution will be confined to a consideration of the purely mechanical model, and will not include any mixed mechanical and electrical systems except in so far as it may be convenient to supply the energy to a working model in electrical or other forms. It is obvious that mechanical models in zoology are, in almost all cases, models of a locomotor system or of other moving parts of those animals which have definable skeletal elements; it is not so obvious that such systems do of necessity rely on certain well defined mechanical principles.
All mechanical devices can be broken down into two elementary systems --levers and wheels. Levers are subdivisible into three types: Levers of the first order, where the fulcrum lies between the power and the weight, as for example in a balance. Levers of the second order, where the weight is between the power and the fulcrum, as in an oar. Levers of the third order, where the power is applied between the fulcrum and the weight, e.g. a sewing-machine treadle or a fishing-rod. It may not be immediately obvious that a wheel and axle in some of its properties may be considered as a lever of the first order. A pulley with a cord wrapped partially round it may be treated as a simple one-to-one lever, but, unlike the simple lever which has a limited angle of swing, the angle of the 'wheel lever' does not change and the movement of the power or weight is unlimited. The lever has two functions, it can alter the direction of application of a force, and it can, by having unequal arms, transform a given force into either a larger force moving through a smaller distance or a smaller force with greater displacement. The foregoing account has described what might be called ideal levers. A practical lever has other properties in addition; its fulcrum and the attachments of the power and weight may all be real pivots with friction. The lever itself cannot be the weightless rigid beam of the ideal system. Apart from these considerations of weight which may cause the actual ratio to differ from the geometrical ratio of the arms, the mass will be important when the lever is accelerated. For a given applied force there is a maximum angular acceleration of a real lever which is set by its own inertia, even when the load is negligible. There is a third factor--elasticity--which may be significant in a model. A practical lever is not rigid and when stressed its shape will change in most cases this distortion will not be appreciable, but it can be of great importance in an oscillating system.