PSYCHOPHYSICAL

PARALLELISM  , in pyschology, the theory that the conscious and nervous processes vary concomitantly whether or not there be any causal connexion between them; in other words " that modifications of consciousness emerge contemporaneously with corresponding modifications of nervous process " (Stout) . The theory is the third possible alternative in considering the relation between mind and body, the others being interaction and one-sided action (e.g. materialism) . It should be observed that this theory is merely a statement, not an explanation . (See PSYCHOLOGY.)which the piston-rod head, or crosshead, as it is often n a'M called, is to be guided . AB C is the middle line of half the beam, C being the fixed centre about which the beam oscillates . A link BD connects a point in the beam with a radius link ED, which oscillates about a fixed centre at E . A point P in BD, taken so that BP.: DP :: EN : CM, move in a path which coincides very closely with the straight line MPN . Any other point F in the line CP or CP produced is made to copy this motion by means of the links AF and FG, parallel to BD and AC . In the ordinary application of the parallel motion a point such as F is the point of attachment of the piston-rod, and P is used to drive a pump-rod . Other points in the line CP produced are occasionally made use of by adding other links parallel to AC and BD . Watt's linkage gives no more than an approximation to straight-line motion, but in a well-designed example the amount of deviation need not exceed one four-thousandth of the length of stroke . It was for long believed that the production of an exact straight-line motion by pure linkage was impossible, until the problem was solved by the invention of the Peaucellier cell .