Just as there are many forms of matter--all springing, we are led to believe, from an undifferentiated, fundamental world-stuff--so there are various kinds of force, reducible, possibly without exception, to one universal principle. Their correlation, indeed, has been already in large measure demonstrated; heat, light, electricity, and kinetic power are known to be equivalent and interchangeable; but there are outstanding activities, which resist assimilation, and seem to originate under different conditions from the rest. Forces manifest themselves chiefly through attractive and repulsive effects, varying in accordance with their natures and the modification of attendant circumstances. The minute particles of matter, for instance, cohere; they cling together tenaciously; yet no pressure avails to bring them into actual contact; at a certain point of mutual approach, they develop an invincible power of resistance to any further encroachments upon their separate molecular domains. And it is this faculty which gives to matter its distinctive property of hardness. It is rendered tangible to sense just by its recalcitrance to constraint. Neither the mode of operation nor the nature of the forces by which molecules are organized into masses is known; while the power acting on the masses thus organized, and regulating by its action the mechanism of the universe, is fully as baffling to comprehension. Wonder at its results is blunted by familiarity; presented to us as novelties, they should be pronounced to outrage reason. The relations of gravity are of the utmost simplicity; and they are, on that very account, supremely perplexing. They are governed by one steadfast law, the same everywhere, and under all varieties of conditions within the range of experiment or precise observation. It governs impartially every kind and quality of matter, taking no notice of its states or combinations, ignoring its subjection to chemical, thermal, magnetic, or electrical influences. Gravity is not only indifferent, but inevitable and inexorable; there is no resisting its sway; no screen serves as a shelter against its persuasions; it spreads equably in all directions, becoming enfeebled, like wave-motion, in the strict proportion of its diffusion from a centre over successive spherical surfaces. Its most singular peculiarity, however, is its apparent unconcern with time. The gravitational pull is virtually instantaneous; its transmission--if it be transmitted--takes place millions of times faster than that of light; the finest tests have, so far, failed to elicit symptoms of delay. These would be found in minute discrepancies between calculated and observed perturbational effects in the heavens.