CHAPTER III 

 MECHANICS AND PHYSICS 



TO build up, as did the Greeks, a scientific 

 astronomy which was altogether different from 

 astrology, is a task which presents very great 

 difficulties ; but when it is a question of explaining 

 physical and mechanical phenomena, these difficulties 

 become almost insurmountable. In this domain we 

 come into collision with such a variety of aspects 

 that it seems impossible to derive them all from a 

 small number of primary notions. 



A badly-hewn tree trunk is in equilibrium on a 

 beam. We feel instinctively that the equal division 

 of the weight round the point of support is the cause 

 of this phenomenon. But how can it be explained 

 accurately ? And is the equality of weights the sole 

 cause ? A bag of sand placed on a bar of iron can 

 remain in equilibrium even if the sand is not equally 

 distributed on the two sides of the bar. 



A piece of deal and a piece of cork of the same size 

 are thrown into the water. The latter sinks less than 

 the former. Is it possible to explain this fact by means 

 of the same theories which make comprehensible the 

 state of equilibrium of the beam or of the bag of 

 sand ? 



Again, it is quite another matter if we pass from the 

 study of bodies at rest or in equilibrium to the study 

 of bodies in motion. We know that a stone falling 

 freely from the height of a tower accelerates its fall. 

 How is this increase of speed to be accurately measured ? 



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