INCOMPLETE PAIRS OF ELEMENTS,
�39. Closure of Pairs of Elements by Sensible Forces.
IN our examination both of the lower and higher pairs of elements, we have hitherto assumed that the reciprocal restraint of the two elements forming the pair was complete ; i.e.. that each of the two bodies by the resistant qualities of its material and the form given to it, both enveloped and constrained the other. We have made this assumption also, expressly or tacitly, in inves- tigating the forms to be given to bodies in order that any sensible forces tending to alter their required relative motion might be balanced by latent forces. Under certain circumstances, however, the strictness of this condition may be somewhat relaxed, when, namely, precautions are taken to prevent the possibility of sensi- ble forces having certain directions ever affecting the pair. If this can be done, it is obvious that it is no longer absolutely necessary to make the pair entirely self-closed, bodily envelopment being no longer essential for restraint in those directions.
In order thus to prevent the disturbing action of sensible forces acting in any given direction upon an element a which is to be con- strained, we allow another sensible force to act upon it continu-