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But to understand the constitution of the material universe we must go a step further, part from the familiar world of sense, and deal with an all-pervading medium, which is at the same time matter and not matter, which lies outside the laws of gravity, and yet obeys other laws intelligible and calculable by us; of which it may be said we know it and we know it not. We call it Ether.

Ether is a medium assumed as a necessary consequence from the phenomena of light, heat, and electricity—primarily from those of light. Respecting light two facts are known to us with absolute certainty.

1st. It traverses space at the rate of 192,000 miles per second.

2nd. It is propagated not by particles actually travelling at this rate, but, like sound through air, by the transmission of waves.

The first fact is known from the difference of time at which eclipses of Jupiter’s satellites are seen according as the earth is at the point of its orbit nearest to or farthest from Jupiter—i.e. from the time light takes to traverse the diameter of the earth’s orbit, which is about 180 millions of miles; and this velocity of light is confirmed by direct experiments, as by noting the difference of time between seeing the flash and hearing the sound of a gun, which gives the velocity of light compared with the known velocity of sound.