The rays of light always proceed in _straight lines_, unless diverted by some intervening body. They are subject to the laws of attraction like other small bodies. If a stream of light be admitted through a small hole into a dark room, and the edge of a knife be applied, it will be diverted from its natural course, and _inflected_ towards it. When the rays of light are thrown back by any opposing body, they are said to be _reflected_. When in passing from one medium to another, they are inflected or diverted from their rectilineal course, they are said to be _refracted_; and this property of light is called its _refrangibility_. Refraction arises from this, that the rays are more attracted by a dense, than by a rare medium. The _velocity_ of light is prodigious, and almost incredible; it moves at the rate of near 200,000 miles in _a second_ of time! Roemer, a Danish philosopher, was the first who found the means of determining the velocity of light, by the difference of time in the eclipses of Jupiter’s satellites, when the earth was on the same, or on the contrary side of the sun, with that planet. This point may be easily proved; for when the earth is between the sun and this planet, those eclipses will happen about 8¼ minutes sooner, than according to the tables; but when the earth is in the contrary position, the eclipses happen about 8¼ minutes later than they are predicted by the tables. Hence, therefore, light takes up about 8¼ minutes in passing from the sun to the earth, a distance of 95,513,794 miles; and it takes about 16½ minutes of time to go through a space equal to the diameter of the earth’s orbit, which is at least 190 millions of miles in length; which is near a million of miles swifter than the motion of a cannon-ball, which flies with the velocity of about a mile in eight seconds.[40] In comparing this velocity of light with that of a cannon-ball, it has been observed, that light passes through a space in about eight minutes, which a cannon-ball with its ordinary velocity, could not traverse in less than thirty-two years! The velocity of sound bears a very small proportion to that of light. Light travels, in the space of eight minutes, a distance in which sound could not be communicated in seventeen years; and even our senses may convince us, if we attend to the explosion of gunpowder, &c, of the almost infinite velocity of the one compared with that of the other.[41] Were the propagation of the rays of light less rapid, the darkness would be very slowly dissipated, and great inconveniences would result to the inhabitants of the earth.