The triply infinite system of null-lines form what is called in linegeometry a complex.
This exquisite familiarity with bird and beast would make us love the memory of Thoreau if his egotism were triply as arrogant, if his often meaningless paradoxes were even more absurd, if his sympathies were even less humanitarian than we know them to have been.
In order to exert force, or at all events that force of reciprocal pressure which we best understand, and on which, in impact, the third law of motion was founded, there are always at least two bodies, enduring, triply extended, mobile, each inert, mutually impenetrable or resistent, different yet similar; and in order to have produced any effect but equilibrium, some bodies must at some time have differed either in mass or in velocity, otherwise forces would only have neutralized one another.
Bodies then are triply extended substances, each occupying enough space to prevent mutual penetration, and by this force of mutual impenetrability or interresistance cause one another to form a joint mass with a common velocity whenever they collide.
He has therefore lost sight of the truths that bodies are triply extended, mutually impenetrable substances, and by this force causes which reduce one another to a joint mass with a common velocity on collision, as for instance in the ballistic pendulum; that these forces are the ones we best understand; and that they are reciprocal causes of the common velocity of their joint mass, whatever happens afterwards.