92 POPULAR LECTURES AND ADDRESSES. 



gravitation unit of matter, and of the reckoning 

 of force adopted by astronomers, in common with 

 all workers in mathematical dynamics, according 

 to which the unit of force is that force, which 

 acting on unit of mass for unit of time, generates 

 a velocity equal to unit of velocity. The universal- 

 gravitation unit of mass is such a quantity of 

 matter, that if two quantities, each equal to it, be 

 placed at unit distance apart, the force between 

 them is unity. 



The universal-gravitation method I refer to 

 for this reason. There is a terrestrial-gravitation 

 reckoning of force, according to the weight of 

 the unit of mass ; and after all, when we terrestrial 

 creatures take a mass in our hand and feel the 

 weight of it, it is a kind of measurement that we 

 cannot do away with. The kilogramme, or the 

 pound, or the ounce, is a thing we have to deal 

 with ; we have it in our hand, and we cannot help 

 using it to give us by its heaviness a reckoning 

 of force. A local gravitation unit of force means 

 the weight of a gramme in London, in Glasgow, 

 at the Equator, or anywhere else and it is 



