ADVANCE OF ASTRONOMY. 181 



The Gravitation Law. — It is necessary, however, 

 to dwell for a little on what is perhaps the greatest 

 of all scientific achievements — Xewton's formulation 

 of the Gravitation Law (1687), — the foundation of 

 what has been called the astronomical view of nature. 

 " Every particle of matter in the universe attracts 

 every other particle with a force whose direction is 

 that of the straight line joining the two, and whose 

 magnitude is proportional directly as the product of 

 their masses, and inversely as the square of their 

 mutual distance '^ — this is the generalisation known 

 as the Law of Gravitation.* Another way of phras- 

 ing it may be quoted : — '^ The law of gravitation 

 states that to each portion of matter we can assign 

 a constant — its mass — such that there is an accelera- 

 tion towards it of other matter proportional to that 

 mass divided by its distance away. Or all bodies 

 resemble each other in having this acceleration to- 

 wards each other." f The fundamental concept is 

 that of mutual acceleration. 



This formula applies with equal accuracy to a 

 stone falling to the ground and to the motion of the 

 earth round the sun. As far as we know, it is uni- 

 versally true. It may not be possible to trace the 

 logical processes of genius, but it should be noted that 

 just as Keppler deduced his three laws from the 

 observations of Tycho Brahe, so Keppler^s laws 

 formed a basis of deduction for E'ewton. 



Summary. — The science of astronomy, most an- 

 cient in its origin, may he said to have 'passed 

 through three main phases — (a) of observation, 

 (b) of analysis and generalisation, and (c) of deduc- 



* Cited from Chambers's Encyclopcedia. 

 t Prof. Poynting, Pres, Address, Section A., Rep. Brit, 

 Ass. for 1899, p. 616. 



