78 GREAT MEN OF SCIENCE 



term weight, and often even size, instead of mass, in reference 

 to the body investigated. The peculiar nature of the con- 

 cept of mass was only cleared up by Newton in the further 

 prosecution of these investigations. 



Research into the processes of collision presented quite 

 peculiar difficulties, though the processes are common 

 enough in everyday life; their peculiarity is that they take 

 place in very short periods of time. Galileo himself already 

 paid attention to collision, and contemporaneously with 

 Huygens (1688), the Englishmen Wallis and Wren (the 

 famous architect), 1 Wallis investigated the collision of 

 inelastic bodies, Wren and Huygens that of elastic bodies, 

 and the essential point is that they found simple relations 

 between the magnitude of the masses concerned in the col- 

 lision and their velocities before and after it, without however 

 needing to investigate the details of the process taking place 

 in the very short period of contact of the two bodies. Huy- 

 gens in this respect was far in advance of the others, and the 

 manner in which he handles the matter gives us remarkable 

 evidence of his gifts as a scientist. In a manner quite 

 new at the time, he connects the discovery already 

 made by Galileo, that velocities of different origin present 

 together do not interfere with one another, with the fact, 

 already used by him in discussing the compound pendulum, 

 that the centre of gravity does not increase in height. From 

 these premises, with the addition of a few experimental facts 

 derived from simple cases of collision, he deduced a large 

 number of the most important laws concerning collision. 

 The agreement of the laws thus found with the experiments 

 carried out upon 'hard' elastic balls, welded the knowledge 

 of the phenomena of motion into a consistent whole. 



In all these investigations of Huygens, we meet for the 



^ Not long afterwards we have Markus Marci in Prague (see Mach, 

 The Science of Mechanics, trans, by T. J. McCormack (4th Ed., 1919). 

 Mariotte's essay on collision, which is more often referred to, came much 

 later (1687). 



