MEDIUM

, the same as mean, either arithmetical, geometrical, or harmonical.

Medium denotes also that space, or region, or fluid, &c, through which a body passes in its motion towards any point. Thus, the air, or atmosphere, is the medium in which birds and beasts live and move, and in which a projectile moves; water is the medium in which fishes move; and æther is a supposed subtile Medium in which the planets move. Glass is also called a Medium, being that through which the rays of light move and pass.

Mediums resist the motion of bodies moving through them, in proportion to their density or specific gravity.

Subtile or Ætherial Medium, is an universal one whose existence is by Newton rendered probable. He makes it universal; and vastly more rare, subtile, elastic, and active than air; and by that means freely permeating the pores and interstices of all other Mediums, and diffufing itself through the whole creation. By the intervention of this subtile Medium he thinks it is that most of the great phenomena of nature are effected. See ÆTHER.

This Medium it would seem he has recourse to, as the first and most remote physical spring, and the ultimate of all natural causes. By the vibrations of this Medium, he supposes that heat is propagated from lucid bodies; as also the intenseness of heat increased and preserved in hot bodies, and from them communicated to cold ones.

By this Medium, he supposes that light is reflected, inflected, refracted, and put alternately into fits of easy reflection and transmission; which effects he also elsewhere ascribes to the power of attraction; so that it would seem, this Medium is the source and cause even of attraction itself.

Again, this Medium being much rarer within the heavenly bodies, than in the heavenly spaces, and growing denser as it recedes farther from them, he supposes this is the cause of the gravitation of these bodies towards each other, and of the parts towards the bodies.

Again, from the vibrations of this same Medium, excited in the bottom of the eye by the rays of light, and thence propagated through the capillaments of the optic nerves into the sensorium, he supposes that vision is performed: and so likewise hearing, from the vibrations of this or some other Medium, excited in the auditory nerves by the tremors of the air, and propagated through the capillaments of those nerves into the sensorium: and so of the other senses.

And again, he conceives that muscular motion is performed by the vibrations of the same Medium, excited in the brain at the command of the will, and thence propagated through the capillaments of the nerves into the muscles; and thus contracting and dilating them.

The elaftic force of this Medium, he shews, must be prodigiously great. Light moves at the rate of considerably more than 10 millions of miles in a minute; yet the vibrations and pulsations of this Medium, to cause the fits of easy reflection and transmission, must be swifter than light, which is yet 7 hundred thousand times swifter than sound. The elastic force of this Medium, therefore, in proportion to its density, must be above 490000 million of times greater than the elastic force of the air, in proportion to its density; the velocities and pulses of the elastic Mediums being in a subduplicate ratio of the elasticities, and the rarities of the Mediums, taken together. And thus may it be conceived that the vibration of this Medium is the cause also of the elasticity of bodies.

Farther, the particles of this Medium being supposed indefinitely small, even smaller than those of light; if they be likewise supposed, like our air, endued with a rcpelling power, by which they recede from each other, the smallness of the particles may exceedingly contribute to the increase of the repelling power, and consequently to that of the elasticity and rarity of the Medium; by that means fitting it for the free transmission of light, and the frce motions of the heavenly bodies. In this Medium may the planets and comets roll without any considerable refistance. If it be 700,000 times more elastic, and as many times rarer, than air, its resistance will be above 600 million times less than that of water; a resistance that would cause no sensible alteration in the motion of the planets in ten thousand years.

MEGAMETER. See Micrometer.