But even these considerations do not present the greatest of the marvels of gravity. It is wonderful, indeed, to consider a form of attraction possessed by the infinitely minute, and exerted over the infinitely vast, operating in portions of time immeasurably small, and extending its operations throughout time infinite. But the mystery of mysteries is not here. The marvel of marvels is this, that, so far as we can perceive, the force of gravity is exerted without any material connection with the objects moved by it. Matter seems to act where it is not, to use the phraseology of the schools. Of this "action at a distance," Newton himself said, that it is inconceivable, that in point of fact it is impossible. "No man," he said, "who has, in philosophical matters, a competent faculty of thinking," can "for a moment believe that a body can act through a vacuum, without the intervention of anything else by or through which the force may be conveyed from one body to another." Yet this is precisely what gravity seems to do. The ether occupies, indeed, all space; but there is nothing at present known to us by which we can understand how the ether can transmit the force of gravity. The power of the ether in the rapid transmission of undulations seems to attain its limit in the propagation of light and heat and electricity at the rate of nearly 200,000 miles per second. How the ether can act so as to serve as a medium of communication between bodies at all distances, transmitting impressions 10,000,000 times faster, at least, than light travels, nothing at present known to us enables us to say. I have, in a lecture which I gave in America upon the mysteries of the universe, indicated a way in which gravity may be conceived to be generated and transmitted; and I may hereafter describe the conception (based partly on the views of Le Sage). But it is only a conception. There is no phenomenon (except the very form of attraction which has to be explained) tending to show that the conception is correct And even if it be accepted, it brings us face to face with only greater marvels. At present, however, let this simply be said in conclusion--that the apparent action of gravity at a distance is, of all physical wonders, the greatest yet known to man. If we accept the opinion of Newton, which, indeed, seems to me indisputable, that matter cannot act through a vacuum, then we must admit the existence of properties, as yet unthought of, in the ether of space, or in some still more subtle universe permeating that ether. If, on the other hand, we accept the belief that matter can act at a distance, then is there no miracle, either of those believed in by mankind generally, or of those more generally rejected, which exceeds in marvellousness this wonder of all the wonders of physical science. FOOTNOTES: