that all Matter is Heavy. 263 



ted by the degree of its expansive or thermometric influence. 

 What scale-beam is more delicate than the thermoscope of Mel- 

 loni? 



15. In the last paragraph but one, seventh page, (p. 270,) you 

 suggest, that ^'perhaps some persons might coficeive that the 

 identity of loeight and inertia is obvious at once, for both are 

 merely resistance to motion ; inertia, resistance to all motion, or 

 change of motion; weight, resistance to motion upwards.''^ 



16. I am surprised that you should think the opinion of any per- 

 son worthy of attention, who should entertain so narrow a view 

 of weight, as antagonist of momentum, as that above quoted, 

 ^^ that it is a resistance to motion upwards.'^ Agreeably to the 

 definition, given at the commencement of the letter, weight, in 

 its usual practical sense, is only one case of the general force 

 which causes all ponderable masses of matter to gravitate 

 towards each other, and which is of course liable to resist any 

 conflicting motion, whatever may be the direction. When in 

 the form of solar attraction, it overcomes that inertia of the 

 planets which would otherwise cause them to leave their orbits, 

 does gravitation " resist motion upivards ?" 



17. In the next paragraph you allege, that " there is a difference 

 in these two kinds of resistance to motion. Inertia is instanta- 

 neous, weight is continuous resistance.^' 



18. It is to this allegation I object, that as you have defined 

 inertia to be ^' resistance to motion, or to change of motion,'''' it fol- 

 lows that it can be instantaneous only where the impulse which 

 it^'esists is instantaneous. It cannot be less continuous than the 

 force by which it is overcome. 



19. Gravity has been considered as acting upon falling bodies by 

 an infinity of impulses, each producing an adequate acceleration; 

 but to every such accelerating impulse, producing of course a 

 ^^ change of motion,''' will there not be a commensurate resist- 

 ance from inertia ? and the impulses and resistances being both 

 infinite, will not one be as continuous as the other ? 



20. I have already adverted to inertia as the continuous antag- 

 onist of solar attraction in the case of revolving planets. 



21. Agreeably to Mossotti, the creation consists of two kinds of 

 matter, of which the homogeneous particles are mutually repel- 

 lent, the heterogeneous mutually attractive. Consistently with 



