The slight numerical discrepancy between the above results is of course due to the approximate character of the data selected, which are taken in round numbers as quite sufficient for purposes of illustration. If we imagine the force applied to the earth by a forest of round rods, one for every square foot of the earth's surface--i.e. of the projected earth's hemisphere or area of equatorial plane,--the force transmitted by each would have to be 2700 tons; and therefore, if of 30-ton steel, they would each have to be eleven inches in diameter, or nearly in contact, all over the earth. _Pull of a Planet on the Earth._ While we are on the subject, it seems interesting to record the fact that the pull of any planet on the earth, even Neptune, distant though it is, is still a gigantic force. The pull of Neptune is 1/20,000th of the sun's pull: i.e. it is 18 billion tons weight. _Pull of a Star on the Earth._ On the other hand, the pull of a fixed star, like Sirius--say a star, for example, which is 20 times the mass of the sun and 24 light years distant--is comparatively very small. It is easily found by dividing 20 times the sun's pull by the squared ratio of 24 years to 8 minutes; and it comes out as 30 million tons weight. Such a force is able to produce no perceptible effect. The acceleration it causes in the earth and the whole solar system, at its present speed through space, is only able to curve the path with a radius of curvature of length thirty thousand times the distance of the star. _Force required to hold together the Components of some Double Stars._ But it is not to be supposed that the transmission of any of these forces gives the ether the slightest trouble, or strains it to anywhere near the limits of its capacity. Such forces must be transmitted with perfect ease, for there are plenty of cases where the force of gravitation is vastly greater than that. In the case of double stars, for instance, two suns are whirling round each other; and some of them are whirling remarkably fast. In such cases the force holding the components together must be enormous.