force arising from their immense velocity, which is computed at nearly two hundred thousand miles in one second.*

When these small particles flowing from a candle, fall upon bodies, and are thereby reflected to our eyes, they excite in us the idea of that body, by forming its image on the retina. † Since bodies are visible on all sides, light must be reflected from them in all directions. A ray of light is a continued stream of these particles, flowing from any visible body in a straight line. That the rays move in straight, and not in crooked lines, (unless they be refracted,) is evident from bodies not being visible if we endeavor to look at them through the bore of a bended pipe; and from their ceasing to be seen by the interposition of other bodies, as the fixed stars, by the interposition of the Moon and planets and the Sun wholly, or in part, by the inter position of the Moon, Mercury or Venus.

There is no physical point, (says Melville,) in the vis ible horizon which does not send rays to every other point; no star in the Heavens, which does not send light to every other star. The whole horizon is filled with rays from every point in it; and the whole visible Universe with a sphere of rays from every star. star. In short, for any thing we know, there are rays of light joining every two physical points in the Universe, and

*Light passes from the Sun to the Earth in 8 minutes and 7 seconds, which is 195,072 miles in one second of time.

† A fine net work membrane, in the bottom of the eye.

that in contrary directions, except when opaque bodies intervene. A ray of light coming from any of the fixed stars to the human eye, has to pass in every part of the intermediate space between the point from which it has been projected, and our solar system, through-rays of light flowing in all directions from every fixed star in the Universe, and in reaching this earth; it has passed across the whole ocean of the solar light, and that which is emitted from the planets, satellites and comets. Yet in this course, its progress has not been intercepted.

The densities and quantities of light, received upon any given plane, are diminished in the same proportion, as the squares of the distances of that planet from the luminous body are increased; and on the contrary, are increased in the same proportion as these squares are diminished.

When a telescope magnifies the disk of the Moon, and planets, they appear more dim than to the bare eye; because the telescope cannot increase the quantity of light in the same proportion that it can magnify the surface, and by spreading the same quantity of light over a given surface, it appears more dim, than when beheld with the naked eye.

When a ray of light passes out of one medium into another, it is refracted, or turned out of its course more or less, as it falls more, or less obliquely on the refracting surface' which divides the two mediums.

In a

This may be proved by several experiments. basin, place a piece of money, or any metalic substance, and then retire from it till the edge of the basin

hides the money from your view, then keeping your head steady, let another pour water gently into the basin, and as the basin fills with the water, more and more of the substance of the bottom will come in sight, and when the basin is filled, the substance at the bottom will be full in view, and appear as if it was lifted up; for the ray which was straight while the basin was empty, is now bent at the surface of the water, and turned out of its natural course into an angular direction, and the more dense the medium is, the more light is reflected in passing through it.

The earth is surrounded by a thin fluid mass of matter, called the AIR, or ATMOSPHERE, which gravitates to the earth, revolves with it in its diurnal motion, and goes with it round the Sun every year. This fluid is of an elastic and springy nature, and that part next the earth being compressed by the weight of all the air above it, is pressed close together, and therefore is the most dense at the surface of the earth, and gradually rarer the higher you ascend.

It is well known, that the air near the surface of our earth possesses a space about nine hundred times greater than water of the same weight, and therefore a cylindric column of air nine hundred feet high, is of equal weight with a cylinder of water of the same diameter one foot high. But a cylinder of air reaching to the top of the atmosphere, (45 miles,) is of equal weight with a cylinder of water about 33 feet high, and therefore, if from the whole cylinder of air, the lower part of nine hundred feet high, is taken away, the upper part

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remaining, will be of equal weight with a cylinder of water 32 feet high. Therefore, at the height of nine hundred feet, the weight of the incumbent air is less, and consequently the rarity of the compressed air is greater, than near the earth's surface in the ratio of 33 to 32.

The weight of the air at the earth's surface, is found, by experiments made with the air pump, and also by the quantity of mercury that the atmosphere balances in the barometer, in which, at a mean state, the mercury stands 29 and inches high. And if the tube were a square inch at the base, and of equal size to the top, it would, at that height, contain 29 and cubic inches of mercury; which is just fifteen pounds, and consequently every square inch of the surface of the earth, sustains a weight of 15 pounds; every square foot 2,160 pounds, at this ratio, and when the mercury is at that height in the barometer, every common sized man sustains a weight of 32,400 pounds, (the area of the surface of his body being about 15 square feet) of air all round; for fluids press equally up and down, and on all sides. But because this enormous weight is equal on all sides, and counterbalanced by the spring of the air diffused through all parts of our bodies, it is not in the least degree felt by us.

The state of the air is such many times, that we feel ourselves languid and dull, which is generally thought to be occasioned by the air's being foggy and heavy about us. But at such times, the air is too light. The truth of this assertion, is known by the sinking of the

mercury in the barometer, and at these times, it is generally found that the air has not sufficient strength to bear up the vapors which compose the clouds; for when it is otherwise the clouds ascend high, and the air is more elastic and weighty about us, and by these means, it balances the internal spring of the air within us, braces the nerves and blood vessels, and makes us brisk and lively.

It is entirely owing to the state of the atmosphere,that the Heavens appear bright even in the day time. For, without an atmosphere, only that part of the Heavens would shine in which the Sun was placed, and if we could live without air, and should turn our backs towards the Sun, the whole Heavens would appear as dark as in the night; and the stars would be seen as clearly as in the nocturnal sky. In this case, we should have no twilight, but a sudden transition from the brightest sunshine to the darkness of night, immediately after Sun-set, and from the blackest darkness to the brightest Sun-shine at Sun-rising.

But, by means of the atmosphere, we enjoy the Sun's light reflected from the aerial particles for some time before he rises, and after he sets. When the earth by its rotation, has withdrawn our sight from the Sun, the atmosphere, (being still higher than we,) has the Sun's light imparted to it which gradually decreases until he has descended 18 degrees below the horizon, and then all that part of the atmosphere which is above us is dark. From the length of twilight, Dr. Heill has calculated the height of the atmosphere (so far as it is sufficiently