Indeed, at present, owing to the more exact observations available, and the greater range of time over which they extend, it may safely be said that the rate of propagation of gravity is far greater than this. It is even held by some that gravity acts instantaneously over any distance, however vast.
Although I cannot here indicate the exact nature of the reasoning by which the enormous rapidity of the action of gravity is inferred, I must briefly indicate the general argument, that the reader may not suppose the matter to be merely speculative. Suppose that the action of gravity were propagated at the same rate as light. Then the earth would feel the pull of the sun eight minutes or so after she had been in the place where the sun began to exert that particular pull. The direction of the pull then would not be that of the straight line connecting the earth and sun at the moment when the pull was felt, but that of the straight line connecting the sun and the earth eight minutes or so before. For instance, when the earth is at E_{ 1}, fig. 6, the sun at S would begin to exert a pull in the line E_{ 1} S, but the earth would only feel this pull when she got to E_{ 2}, her place eight minutes later, when it would act upon her in the direction E_{ 2} F, parallel to E_{ 1} S. Now this pull, E_{ 2} F, may be divided into two parts, one along E_{ 2} S, pulling the earth towards the sun S, the other along E_{ 2} T in the earth's course, _hastening her therefore_. But the maintenance by the earth of the same constant track depends entirely on the action of gravity sunwards. If there is any action in addition, hastening the earth, then she will not keep her course,[6] but will travel in a constantly widening path,--or, in a sort of spiral, very slowly retreating from the sun, but retreating constantly. The change of distance would not be measurable in millions of years; but the increase in the length of the year _would_, before long, be observable. Because there is no such increase, astronomers feel well assured that gravity is not only propagated more swiftly than light, but many times, even, as we have seen, many millions of times, more swiftly.
It is then in an infinitely minute time that the action of gravity traverses all ordinary distances. The earth's pull on the moon takes less than the 50,000,000th part of a second in reaching the moon,--and the particles constituting the mass of the earth act on ourselves, and on all the objects which lie near the earth's surface, in far less than the 10,000,000th part even of this utterly minute time-interval.