Among small bodies gravitational forces are slight, and are altogether exceeded by magnetic and electric or chemical forces. Indeed gravitational attraction between bodies of a certain smallness can be more than counterbalanced even by the pressure which their mutual radiation exerts--almost infinitesimal though that is;--so that as a matter of fact, small enough bodies of any warmth will repel each other unless they are in an enclosure of constant temperature, i.e. unless the radiation pressure upon them is uniform all round. The size at which radiation repulsion over-balances gravitational attraction, for equal spheres, depends on the temperature of the spheres and on their density; but at the ordinary temperature to which we are accustomed, say 60° Fahrenheit or thereabouts, equality between the two forces will obtain for two wooden spheres in space if each is about a foot in diameter; according to Professor Poynting's data (_Philosophical Transactions_, Vol. 202, p. 541). For smaller or hotter bodies, radiation repulsion overpowers mutual gravitation; and it increases with the fourth power of their absolute temperature. The gravitational attractive force between particles is exceedingly small; and that between two atoms or two electrons is negligibly small, even though they be within molecular distance of each other. For instance, two atoms of, say, gold, at molecular distance, attract each other gravitationally with a force of the order γ (10⁻²² x 10⁻²²) / (10⁻⁸)² = 10⁻⁴⁴ / 10⁻¹⁶ x 10⁻⁷ = 10⁻³⁵ dyne; which would cause no perceptible acceleration at all. The gravitational attraction of two electrons at the same distance is the forty-thousand-millionth part of this, and so one would think must be entirely negligible. And yet it is to the aggregate attraction of myriads of such bodies that the resultant force of attraction is due;--a force which is felt over millions of miles. The force is not only felt indeed, but must be reckoned as one of prodigious magnitude. When dealing with bodies of astronomical size, the force of gravitation overpowers all other forces; and all electric and magnetic attractions sink by comparison into insignificance. These immense forces must be transmitted by the ether, and it is instructive to consider their amount. SOME ASTRONOMICAL FORCES WHICH THE ETHER HAS TO TRANSMIT. _Arithmetical Calculation of the Pull of the Earth on the Moon._ The mass of the earth is 6000 trillion (6 × 10²¹) tons. The mass of the moon is 1/80th that of the earth. Terrestrial gravity at the moon's distance (which is 60 earth radii) must be reduced in the ratio 1:60²; that is, it must be 1/3600th of what it is here. Consequently the pull of the earth on the moon is 6 × 10²¹ / 80 × 3600 tons weight. A pillar of steel which could transmit this force, provided it could sustain a tension of 40 tons to the square inch, would have a diameter of about 400 miles; as stated in the text, page 102.