THE PRESIDENT'S ADDRESS 29 



able to discover, pushed their study of moving bodies beyond 

 the mere 'consideration of change of position. None of them 

 had recognized the inertia of the moving body as a funda- 

 mental — perhaps the fundamental-fact of mechanics. Princes 

 and paupers, for ages, had stumped their toes against bricks 

 and stones: they were doubtless quite as familiar as we with 

 the mere fact of inertia. But to Galileo it was a cardinal 

 fact, because he was the first to see that the future history 

 of a body depends upon its possession of inertia. To him the 

 importance of a motion is. in general, measured by the inertia 

 involved, or as was then said — the weight involved. Hence 

 he assigned to the product of the weight and velocity of a 

 body the name "momentum." which is merely the Latin 

 word for importance : as a synonym he sometimes uses the 

 word impetus, thus emphasizing the impetuosity of motion. 



But Galileo never got beyond the point where he meas- 

 ured inertia by weight, as. indeed, engineers still do — all. at 

 least, except electrical engineers. The invention of the idea 

 of mass was reserved for Xewton. Even Huygens.l 10) who 

 first mastered the idea of centrifugal force, never got beyond 

 the point where he measured centrifugal forces in terms 

 of weight, thus avoiding the conception of mass in all his 

 work. 



Those who wish to see just how clearly Galileo conceived 

 that the future behavior of a body is connected with its inertia 

 should read those propositions in his "Mechanics" (11) in 

 which he calculates the path of a projectile by assuming 

 that the horizontal speed of a shot, after it has left the muz- 

 zle of a gun. continues to be uniform. His repeated use of 

 this principle makes it perfectly clear that he discovered what 

 we now call — and perhaps properly call — Newton's first law 

 of motion. Galileo failed to generalize it by extending it to 

 all bodies whether subject to the earth's gravitation or not. 

 This Xewton did because he had acquired the new concept of 

 mass — that constant property which never deserts a body in 

 anv position or condition. 



3. The next great step which Galileo made was the dis- 

 covery of the constant factor in the motion of falling bodies. 

 One of his earliest experiments, performed while still a young 

 man at the University of Pisa, was to allow a bronz ball to roll 

 down a perfectly prepared inclined plane, an experiment from 

 which he cleverly inferred that while the position and speed 

 of the ball were changing, the time-rate at which it gained 

 momentum remained constant. It was with reference to 

 these particular experiments that Goethe remarked "dem 

 Genie, ein Fall fur tausend gelte." The experiment is com- 

 pleted by showing how one can compute the momentum (or 



