278 Professor John H. Poynting [Feb. 2i 



WEEKLY EVENING MEETING. 

 Friday, February 23, 1900. 



His Grace The Duke of Northumberland, K.G. F.S.A. 

 President, in the Chair. 



Professor John H. Poynting, D.So. F.E.S. 



Becent Studies in Gravitation. 



The studies in gravitation which I am to describe to you this 

 evening will perhaps fall into better order if I rapidly run over the 

 well beaten track which leads to those studies, the track first laid 

 down by Newton, based on astronomical observations, and only made 

 firmer and broader by every later observation. 



I may remind you, then, that the motion of the planets round the 

 sun in ellipses, each marking out the area of its orbit at a constant 

 rate, and each having a year proportional to the square root of the 

 cube of its mean distance from the sun, implies that there is a force 

 on each planet exactly proportioned to its mass, directed towards, 

 and inversely as the square of its distance from the sun. The lines 

 of force radiate out from the sun on all sides equally, and always 

 grasp any matter with a force proportional to its mass, whatever 

 planet that matter belongs to. 



If we assume that action and reaction are equal and opposite, 

 then each planet acts on the sun with a force proportional to its own 

 mass ; and if, further, we suppose that these forces are merely the 

 sum totals of the forces due to every particle of matter in the bodies 

 acting, we are led straight to the law of gravitation, that the force 

 between two masses M. 1 M 2 is always proportional to the product of 

 the masses divided by the square of the distance r between them, or 

 is equal to 



G x M! x M 2 



and the constant multiplier G is the constant of gravitation. 



Since the force is always proportional to the mass acted on, and 

 produces the same change of velocity whatever that mass may be, the 

 change of velocity tells us nothing about the mass in which it takes 

 place, but only about the mass which is pulling. If, however, we 

 compare the accelerations due to different pulling bodies, as for 

 instance that of the sun pulling the earth, with that of the earth 

 pulling the moon, or if we compare changes in motion due to the 

 different planets pulling each other, then we can compare their 

 masses and weigh them, one against another and each against the 



