Sentence:  And if the mass m, and the <WSD>restitutional</WSD> force were such that, at a foot from the centre, there was produced a <WSD>restitutional</WSD> force of F poundals {i.e., a <WSD>restitutional</WSD> force capable in one second of producing in a mass of 1 lb., an acceleration of F feet per second), then the time of vibration would be [...] 
Meaning of the word marked with <WSD> tags:  Pertaining to or exhibiting restitution (return or restoration to a previous condition or position).