462 
its power for evil destroyed. Ifthe mechan- 
ism of attraction were not concealed, or if 
it had some distinguishing mark other than 
acceleration, and it were possible to experi- 
ment with it as with springs, such an error 
could not be made, even with improper 
axes of reference. 
A convenient way of regarding the laws 
of motion is to consider, as before, the 
second law as affirming the relation be- 
tween force and change of momentum, and 
the third and first as asserting the princi- 
ple of conservation of momentum ; the third 
implying that momentum passes from one 
body to another without change, and the 
first that the only way by which the mo- 
mentum of a body can suffer alteration is 
by part of it passing into another body. 
Again, if it be assumed that the third law 
implies that action and reaction are in the 
same straight line the principle of the con- 
servation of angular momentum will follow. 
The statement is sometimes made that 
the ‘bodies’ of Newton’s laws must be 
regarded as particles. I cannot take this 
view ; they are real bodies, of all sizes, and 
with all the qualities known and unknown 
of such bodies. They are not the imaginary 
bodies of the mathematician, the dramatis 
persone of the algebraic theatre, possessing 
only the qualities arbitrarily assigned to 
them for the special purpose of the investi- 
gation in hand. 
The laws of dynamics thus hold for all 
bodies within the solar system whose 
masses, forces and motions have hitherto 
been observed and measured; but the 
motions must be measured with essential 
reference to only one set of axes, namely, a 
set whose origin is in the sun and whose 
directions are fixed by the stars. 
Kinematics deals with relative motion; 
Dynamics with the ‘Motus Absolutus’ of 
the Principia. 
We now pass to the consideration of the 
laws of energy in their dynamical relations. 
SCIENCE. 
[N.S. Von. VI. No. 143. 
In the discussion of statics as the fore- 
runner of dynamics, attention was directed 
mainly to the springs and strings and 
weights by which the forces were measured. 
The original statical experiments may also 
be regarded as the source of the principles of 
energy in connection with mechanical sci- 
ence. From this point of view the bodies 
upon which the forces act come into promi- 
nence, not because of their masses as in dy- 
namics, but on account of their shapes, sizes 
and rigidity. Thus the experiments were 
made with levers, pulleys, inclined planes, 
wedges, etc.—in fact, with instruments for 
doing work, the mechanical powers of the 
text-books. In the statical principle of vir- 
tual velocities we have the origin of the prin- 
ciple of the equivalence of work and energy. 
To men of all times the most natural way 
of regarding force has been, as the action 
by which material is stretched, bent, 
twisted, broken or displaced, 7. e., whereby 
work is done. Even the word momentum, 
in the language of ordinary life, implies the 
power of doing work. It is worth consid- 
eration whether it may not be better in the 
instruction of students to work up to the 
ideas of dynamics through elementary ex- 
amples of the equivalence of work and ki- 
netic energy, rather than by taking the 
ordinary balloon passage to the laws of mo- 
tion. While less systematic and formal, 
this procedure would be more natural and 
probably more useful. 
The laws of energy may be summarized 
as follows. When work is done on a body 
an equivalent amount of energy is partly 
transformed and partly transferred without 
transformation. It is in general partly 
transmitted to other bodies with which the 
given body may be in physical connection. 
Its transformations are into stored energy 
and dissipated energy. Examples of 
stored energy are the potential energies 
due to gravitation, the forces of elasticity, 
magnetic and electrical attractions and 
