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may be only geometrical, or it may be kinematic, or dynamic. Two 

 structxores are said to be geometrically similar if the ratios of 

 all homologous dimensions are equal. Kinematic similarity is 

 similarity of motion. The motions of kinematically similar sys- 

 tems are geometrically similar and the ratios of the velocities 

 of the several homologous particles in motion are equal. Few 

 models are similar to a higher degree. It is possible, however, 

 for two systems to possess dynamic similarity either in certain 

 or all respects. In order to achieve dynamic similarity the two 

 systems must be kinematically similar and in addition the ratios 

 of the homologous masses and forces concerned iji the motion must 

 be equal. The condition of complete dynamic similarity is seldom 

 achieved except in instances where the model is identical with the 

 prototype in every respect. 



