A HISTORY OF SCIENCE 



whether this force which operates so tangibly here at 

 the earth's surface may not extend its influence out 

 into the depths of space, so as to include, for example, 

 the moon. Obviously some force pulls the moon con- 

 stantly towards the earth; otherwise that body would 

 fly off at a tangent and never return. May not this 

 so-called centripetal force be identical with terrestrial 

 gravitation? Such was Newton's query. Probably 

 many another man since Anaxagoras had asked the 

 same question, but assuredly Newton was the first man 

 to find an answer. 



The thought that suggested itself to Newton's mind 

 was this : If we make a diagram illustrating the orbital 

 course of the moon for any given period, say one 

 minute, we shall find that the course of the moon de- 

 parts from a straight line during that period by a 

 measurable distance that is to say, the moon has been 

 virtually pulled towards the earth by an amount that 

 is represented by the difference between its actual 

 position at the end of the minute under observation 

 and the position it would occupy had its course been 

 tangential, as, according to the first law of motion, it 

 must have been had not some force deflected it towards 

 the earth. Measuring the deflection in question 

 which is equivalent to the so-called versed sine of 

 the arc traversed we have a basis for determining 

 the strength of the deflecting force. Newton con- 

 structed such a diagram, and, measuring the amount 

 of the moon's departure from a tangential rectilinear 

 course in one minute, determined this to be, by his 

 calculation, thirteen feet. Obviously, then, the force 

 acting upon the moon is one that would cause that 



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