SEPTEMBER 24, 1897. ] 
molecular forces. Such forces are termed 
conservative. Kinetic energy is another 
form of stored energy. Energy is dissi- 
pated by means of the forces of viscosity 
and friction, known as dissipative forces. 
Energy is also stored and dissipated in cer- 
tain electrical, electro-magnetic, thermal, 
chemical and other actions which have not 
been identified with force and which, there- 
fore, are not dynamical. 
In order that work may be done there 
must be a source of energy, or place from 
which it comes, and a sink, or place to which 
it goes, together with an energy stream 
from the source to the sink. When work 
is done continuously the energy stream is 
accompanied by a circuit or system of 
stored energy which acts automatically as 
a moderator of its fluctuations. 
The principle of conservation affirms that 
energy can neither be created nor destroyed, 
so that its changes are changes in form but 
not in amount. 
The principle of the equivalence of work 
and energy is analogous to the second law 
of motion, considered as expressing the 
equivalence of impulse and momentum; that 
of the conservation of energy has its ana- 
logue in the third and first laws of motion 
regarded as affirming the conservation of 
momentum. 
Newton notices this analogy in his scho- 
lium to the laws of motion in the words, 
‘just as bodies in cases of collision have 
the same effect, whose velocities are in- 
versely as their masses, so in putting ma- 
chines in motion agents have the same 
effect, whose velocities in the directions of 
their forces are inversely as these forces.’ 
The now well known reference in the same 
scholium to the action of machines, the im- 
portance of which was pointed out in 
Thomson and Tait’s Natural Philosophy, 
was in continuation of the same line of 
thought. 
The impulse or time integral of a force 
SCIENCE. 
463 
is fully accounted for by the change of mo- 
mentum, while the work or displacement 
integral is only partially accounted for by 
the change in kinetic energy, in all cases of 
real bodies. The reason for the difference 
is that the laws of motion are a complete 
statement of our experience of force in re- 
lation to the motion of a body as a whole, 
i. €., the motion of its center of mass. On : 
the other hand, the laws of energy require 
the consideration not only of this motion, 
but also of all internal motions and forces. 
The principle of the equivalence of work 
and energy is a statement of an effect of 
force essentially different from its effect in 
producing change of momentum. It might 
be supposed, therefore, that this princi- 
ple would be useful in affording another 
means of measuring force. The impossi- 
bility, in general, of measuring the whole 
change of energy due to an unknown force 
acting through an observed distance ren- 
ders this idea to a great extent fruitless. 
If the laws of energy are true such a 
method of measuring force must give the 
same result as the dynamical method. The 
measurement of force by springs is based 
on this principle, and not on the second 
law of motion. Although no attempt is 
made to measure the change of energy due 
to the work of extending a spring, yet ex- 
perience goes to show that the energy 
changes due to given extensions made in 
the same order are constant, and hence the 
corresponding forces are constant. 
The connection between the laws of en- 
ergy and those of motion may be stated as 
follows: Energy and work, like force, are 
fundamental conceptions gained from expe- 
rience and having various relations with 
phenomena which can be discovered only 
as a result of experiment and observation. 
One of these relations is that work is pro- 
portional to the product of force into dis- 
placement. This relation is, therefore, a 
natural law, of the same order of impor- 
