Pièce 74 - Letter from John Herschel

4 pp

Collingwood - JH does not think his views differ very much from [Robert L.] Ellis: 'I readily accept his 'hierarchy of causes' and I am quite willing to receive mechanical force as the cause ultimately set in action'. Bodies affect our senses by 'impressing mechanical movements in the nerves'. By 'qualitative action' JH meant 'changes induced on the exercise of forces among the molecules of bodies - alterations of their dynamical energies which alterations I conceive cannot be the result of mere mechanical force which can but push and pull a particle but cannot alter its power to push and pull another, either temporarily or permanently'. JH thinks it likely that when a copper wire acts on a magnetic needle, a power of attraction and repulsion may be transcently communicated to its molecules. Ellis's views 'that one molecule of matter may communicate to another properties itself possesses falls in very well with this and thus power may be propagated along a chain of molecules', but he does not see what the relevance of A's motion to B 'has to do with A's power to impart to B a power to exert force'. Faraday's 'inductive action of magnetic currents effectually destroys the usual dynamical relation between force time and velocity for it makes the force by which a particle A of one wire acts on a particle B of another dependent on the relative velocity and direction of A's motion with respect to B'.