50 
IOWA ACADEMY OF SCIENCES. 
tation also could be accounted for on a similar hypothesis, and 
that the properties required for the propagation of gravitation 
are similar to those exhibited by the ether in the phenomena of 
light and electricity. This theory is the only one that is in 
harmonj^ with what is known of both gravitation and the ether. 
It is simple, and makes no assumptions whatever regarding the 
nature of matter or of atoms. It is incomplete in that it leaves 
the nature of the strain undetermined. 
The non-polar character of gravitation, its symmetry in 
every way about the atom, reduces to two the possible kinds of 
strain required by Maxwell’s hypothesis. These are displace- 
ments of ether radially (1) outward from or (2) inward toward 
the atom. Assuming, as is customary, that the ether is incom- 
pressible, the radial displacement over a spherical surface 
about the atom is constant; and therefore the displacement and 
the intensity of the stress at any point varies inversely as the 
square of its distance from the atom. It is not necessary to 
suppose, either, that the atom itself is spherical or that the dis- 
placements in its immediate vicinity are directed toward or 
from a single point; for at the distance of a single centimeter 
from the atom the surface of equal displacement must be so 
nea^rly spherical that the most accurate observation nov/ possi- 
ble would fail to detect anj^ irregularity. Possibly variations 
in the form of the atom or in the direction of displacement 
immediately around it may be the cause of the chemical proper- 
ties of the atom, since these are apparent only at very small 
distances from it. 
For the sake of clearness let us suppose that outward dis- 
placement of the ether is caused by the insertion of a quantity 
of matter, an atom, at any point. Draw a cone having the cen- 
ter of displacement for its vertex. Any small element in this 
cone is by its outward displacement shortened and widened; so 
that there is on each end of the conical element a pressure, and 
in all directions perpendicular to the pressure a tension due to 
the stretching of the expanded spherical shell containing the 
element. 
Suppose, also, for the sake of clearness, that inward dis- 
placement is produced either by cutting out small portions of 
the ether and leaving holes (atoms) toward which the strain is 
directed, or by condensing small portions of the ether into 
atoms. An element of the cone is by its inward displacement 
lengthened and made narrower, and has a tension on each end 
and a pressure in all directions perpendicular to the tensions. 
