COOPERATION IN SCIENCE 111 



compared the force requisite to keep the moon in 

 her orb with the force of gravity at the surface of 

 the earth, and found them answer pretty nearly." 

 As early as March of that same year Hooke had 

 communicated to the Society an account of experi- 

 ments in reference to the force of gravity at differ- 

 ent distances from the surface of the earth, either 

 upwards or downwards. At this and at every point 

 in Newton's discovery the records of co-workers are 

 to be found. 



By Flamsteed, the first Royal Astronomer, were 

 supplied more accurate data for the determination of 

 planetary orbits. To Huygens Newton was indebted 

 for the laws of centrifugal force. Two doubts had 

 made his meticulous mind pause one, of the ac- 

 curacy of the data in reference to the measurement 

 of the meridian, another, of the attraction of a spher- 

 ical shell upon an external point. In the first matter 

 the Royal Society, as we have seen, had been long 

 interested, and Picard, who had worked on the 

 measurement of the earth under the auspices of the 

 Academic des Sciences, brought his results, which 

 came to the attention of Newton, before the Royal 

 Society in 1672. The second difficulty was solved 

 by Newton himself in 1685, when he proved that a 

 series of concentric spherical shells would act on an 

 external point as if their mass were concentrated at 

 the center. For his calculations henceforth the plan- 

 ets and stars, comets and all other bodies are points 

 acted on by lines of force, and " Every particle of 

 matter in the universe attracts every other particle 

 with a force varying inversely as the square of their 

 mutual distances, and directly as the mass of the 



