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 

 harmony 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 

 nearly spherical that the most accurate observation now possi- 

 ble would fail to detect any 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. 



