Feb., 1889. 
spencer’s “first principles.” 
38 
THE FOUNDATIONS OF OUR BELIEF IN THE 
INDESTRUCTIBILITY OF MATTER AND THE 
CONSERVATION OF ENERGY. 
A CRITICISM OF SPENCER’S “FIRST PRINCIPLES,” 
Paht II., Chapters IV., V., and VI. 
BY J. H. POYNTING, D.SC., F.R.S. 
(Concluded from page 11.) 
Passing on from this, let us consider another experiment: 
that of the swinging pendulum described by Mr. Spencer. 
And to begin with, let us suppose that I set it swinging with a 
blow. It starts off in rapid motion, but as it rises up the 
motion gets less and less and ultimately ceases; only how¬ 
ever for a moment. Back it comes on the return journey, and, 
when once more at the starting point, it is moving as nearly 
as I can judge at the original rate. Again it rises up, now on 
the other side, and with speed slackening till it stops ; again 
it returns and so on, the oscillation continuing though the 
motion is intermittent. As Spencer points out, the motion of 
the pendulum is the objective correlate of our sense of 
muscular effort experienced in starting it, not, however, mere 
effort like the effort of holding a weight in a given position, 
but of muscular effort combined with motion, for we pushed 
the hand along in giving the pendulum the blow. It is 
unfortunate that we have no single sensation which we 
naturally correlate with the combination of effort and motion, 
but we all have the idea fixed firmly enough in our minds as 
work, and this is shown by the common use of the term. To 
take a familiar example. If bricks have to be carried up a 
scaffold, the work done is naturally measured by the weight 
of bricks multiplied by the height of the scaffold ; or we think 
of this product, force x distance, as describable by the single 
term Work. Hence we say the motion of the pendulum is 
the objective correlate of the work done by us. Now as the 
motion disappears, does it go out of existence ? and as it 
begins again, does it start afresh ? Our continuity or 
identity postulate is ready at our elbows to suggest identity of 
the motion in succeeding oscillations, and we have a con¬ 
firmation of the suggestion that it still exists, even when it 
disappears as motion, in the fact that if at the top of the 
swing we lay hold of the pendulum it pulls at our hands ; 
it is ready in fact to give back to us work such as we gave to 
it. We conclude then that there is a continuity of existence, 
at one time showing itself as motion, and at another 
