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that all Matter is Heavy. — 263 
ted by the degree of its expansive or thermometric. influence. 
eg scale-beam is more delicate than the thermoscope of Mel- 
loni 
a In the last paragraph but one, seventh page, (p. ay) you 
suggest, that ‘“‘ perhaps some persons might conceive 
identity of weight 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 quotedy 
“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, 
gh gravitation “resist motion upwards 1” 
. In the next paragraph you allege, that “there is a difference 
in piles two kinds of resistance to motion. Inertia ts instanta- 
neous, weight is continuous resistance.” 
18. It is to this allegation I object, that as you have defined 
inertia to be “‘reststance to motion, or to change of motion,” it fol- 
lows that it can be instantaneous only where the impulse which 
it resists 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 ot 
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 
