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light. Has there been really that almost infinite progress, of 
which Dr. Tyndall speaks, beyond Newton and Leibnitz and 
the students of last century ? Have the present generation 
of physical students, by virtue of these doctrines, a far deeper 
insight into the true system of nature than their predecessors 
could ever attain ? This, I believe, is a grand illusion, fraught 
with no small degree of moral mischief. Analysts have made 
some real advance in dealing with various dynamical problems. 
Observation and experiment have unfolded more clearly the 
connection between diverse forms of physical change, usually 
expressed by different names. But along with this advance 
there is great danger, what with the coinage of new phrases 
for old ideas, and free scientific guess-work, of going backward 
instead of forward. Already, in more cases than one, mere 
verbiage, or even direct contradictions, have been palmed 
on the credulous as grand experimental discoveries, or still 
more grand d priori truths. 
What, then, is this Energy, about which such great dis- 
coveries have been made? Few of those who speak or write 
about it seem to have settled clearly what they mean by the 
term. Is it force or motion ? Is it both or is it neither, being 
something quite distinct from both ? All these four opinions 
seem to be held, and by writers of some eminence. According 
to Mr. Spencer, it is force, and the better name for the con- 
servation of energy is the persistence of force. According 
to Mr. Grove it is motion, and the various forms of energy 
are “ modes of motion.” According to Professors Thomson 
and Tait, who understand the subject better, it is both, or 
rather it is each in turn. It is of two kinds, potential and kinetic. 
The first is an integral of forces, such as have acted or will 
act, when a system passes from a first to a second position. 
Kinetic energy is an integral of velocities or motions, or their 
total amount from zero up to the actual values at any given 
time. These are three varieties ; that it is force, motion, or 
partly one, partly the other. Mr. Brooke adds a fourth 
variety, that it is neither force nor motion, but some- 
thing, distinct from both. While he distinguishes it from 
force, he also inverts the use of the two terms. His Energy is 
exactly the same as the Force of Newton's definition, and of 
nearly every work on dynamics; while his Force is the 
Potential Energy of Sir W. Thomson's analytical theory. 
According to Mr. Spencer, the Conservation of Energy, or 
as he prefers to call it, the Persistence of Force, is the chief 
and foremost of all d priori truths. It holds in his philo- 
sophy exactly the same place as the Being of God in the 
