324 
NATURE 
ception of dines of force which, in his hands, became the 
key to the science of electricity. 
When we observe one body acting on another at a 
distance, before we assume that this action is direct and 
immediate, we generally inquire whether there is any 
material connection between the two bodies, and if we 
find strings, or rods, or mechanism of any kind, capable 
of accounting for the observed action between the bodies, 
we prefer to explain the action by means of these inter- 
mediate connections, rather than to admit the notion of 
direct action at a distance. 
Thus when we ring a bell by means of a wire, the suc- 
cessive parts of the wire are first tightened and then 
moved, till at last the bell is rung at a distance by a pro- 
cess in which all the intermediate particles of the wire 
have taken part one after the other. We may ring a bell 
at a distance in other ways, as by forcing air into a long 
tube at the other end of which is a cylinder with a piston 
which is made to fly out and strike the bell. We may 
also use a wire, but instead of pulling it, we may connect it 
at one end with a voltaic battery, and at the other with 
an electro-magnet, and thus ring the bell by electricity. 
Here are three different ways of ringing a bell. They 
all agree, however, in the circumstance that between the 
ringer and the bell there is an unbroken line of commu- 
nication, and that at every point of this line some physi- 
cal process goes on by which the action is transmitted 
from one end to the other. The process of transmission 
is not instantaneous, but gradual, so that there is an in- 
terval of time after the impulse has been given to one 
extremity of the line of communication, during which 
the impulse is on its way, but has not reached the other 
end. 
It is clear, therefore, that in many cases the action 
between bodies at a distance may be accounted for by a 
series of actions between each successive pair of a series 
of bodies which occupy the intermediate space, and it is 
asked, by the advocates of mediate action, whether, in 
those cases in which we cannot perceive the intermediate 
agency, it is not more philosophical to admit the existence 
of a medium which we cannot at present perceive, 
than to assert that a body can act at a place where it is 
not. 
To a person ignorant of the properties of air, the 
transmission of force by means of that invisible medium 
would appear as unaccountable as any other example of 
action at a distance, and yet in this case we can explain 
the whole process, and determine the rate at which the 
action is passed on from one portion to another of the 
medium. 
Why then should we not admit that the familiar mode 
of communicating motion by pushing and pulling with 
our hands is the type and exemplification of all action 
between bodies, even in cases in which we can observe 
nothing between the bodies which appears to take part in 
the action ? 
Here for instance is a kind of attraction with which 
Prof. Guthrie has made us familiar. A disc is set in 
vibration, and is then brought near a light suspended 
body, which immediately begins to move towards the disc 
as if drawn towards it by an invisible cord. What is this 
cord? Sir W. Thomson has pointed out that in a moving 
fluid the pressure is least where the velocity is greatest. 
The velocity of the vibratory motion of the air is greatest 
nearest the disc. Hence the pressure of the air on the 
suspended body is less on the side nearest the disc 
than on the opposite side, the body yields to the greater 
pressure, and moves toward the disc. 
The disc, therefore, does not act where it is not. It 
sets the air next it in motion by pushing it, this motion 
is communicated to more and more distant portions of 
the air in turn, and thus the pressures on opposite sides 
of the suspended body are rendered unequal, and it moves 
towards the disc in consequence of the excess of pressure. | 
-weight. 
The force is therefore a force of the old school—a case of 
vis a tergo, a shove from behind, 
The advocates of the doctrine of action at a distance, 
however, have not been put to silence by such arguments. 
What right, say they, have we to assert that a body 
cannot act where it is not? Do We not see an instance 
of action at a distance in the case of a magnet, which 
acts on another magnet not only at a distance, but with 
the most complete indifference to the nature of the matter 
which occupies the intervening space? If the action 
depends on something occupying the space between the 
two magnets, it cannot surely be a matter of indifference 
whether this space is filled with air or not, or whether 
wood, glass, or copper, be placed between the magnets. 
Besides this, Newton’s law of gravitation, which every 
astronomical observation only tends to establish more 
firmly, asserts not only that the heavenly bodies act on 
one another across immense intervals of space, but that 
two portions of matter, the one buried a thousand miles 
deep in the interior of the earth, and the other a hundred 
thousand miles deep in the body of the sun, act on one 
another with exactly the same force as if the strata 
beneath which each is buried had been non-existent. 
If any medium takes part in transmitting this action, it 
must surely make some difference whether the space 
between the bodies contains nothing but this medium, or 
whether it is occupied by the dense matter of the earth or 
of the sun, 
But the advocates of direct action at a distance are not 
content with instances of this kind, in which the pheno- 
mena, even at first sight, appear to favour their doctrine. 
They push their operations into the enemy’s camp, and 
maintain that even when the action is apparently the 
pressure of contiguous portions of matter, the contiguity 
is only apparent—that a space a/ways intervenes between 
the bodies which act on each Other. They assert, in 
short, that so far from action at a distance being im- 
possible, it is the only kind of action which ever occurs, 
and that the favourite old ws d@ tevgo of the schools has 
no existence in nature, and exists only in the imagination 
of schoolmen. 
The best way to prove that when one body pushes 
another it does not touch it, is to measure the dis- 
tance between them, Here are two glass lenses, one 
of which is pressed against the other by means of a 
By means of the electric light we may obtain on 
the screen an image of the place where the one lens 
presses against the other. A series of coloured rings is 
formed on the screen. These rings were first observed 
and first explained by Newton. The particular colour of 
any ring depends on the distance between the surfaces 
of the pieces of glass. Newton formed a table of the 
colours corresponding to different distances, so that by 
comparing the colour of any ring with Newton’s table, we 
may ascertain the distances between the surfaces at that 
ring. The colours are arranged in rings because the sur- 
faces are spherical, and therefore the interval between the 
surfaces depends on the distance from the line joining 
the centres of the spheres. The central spot of the 
rings indicates the place where the lenses are nearest 
together, and each successive ring corresponds to an 
increase of about the 4,oooth part of a millimetre in the 
distance of the surfaces. i 
The lenses are now pressed together with a force equal 
to the weight of an ounce, but there is still a measurable 
interval between them, even at the place where they are 
nearest together. They are not in optical contact. To 
prove this, I apply a greater weight. A new colour appears 
at the central spot, and the diameters of all the rings in- 
crease. This shows that the surfaces are now nearer than at 
first, but they are not yet in optical contact, for if they were, 
the central spot would be black. I therefore increase 
the weights, so as to press the lenses into optical contact. 
But what we call optical contact is not real contact. 
[ Fed. 27, 1873, 
