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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.

This may be proved by several experiments. In a basin, place a piece of money, or any metallic 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 it 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. [Plate 6th, fig. 9th.]

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 remaining, will be of equal weight with a cylinder of water 32 feet high. Wherefore, at the height of nine hundred feet, the weight of the incumbent air is less, and consequently the marity

of the compressed air is greater than near the earth's surface in the ratio of 33 to 32.

The weight of the air on 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 a half 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 a half 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 vapours 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

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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 sunset, and from the blackest darkness to the brightest sunshine 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. Reill has calculated the height of the atmosphere (so far as it is sufficiently dense to reflect any light) to be about forty-four miles high.But it seldom is sufficiently dense at two miles' height to bear up the clouds.

The atmosphere refracts the Sun's rays so as to bring him in sight every clear day before he rises in the horizon, and to keep him in view for some minutes after he is really set below it. For, at some times of the year, we see the Sun ten minutes longer above the horizon than he would be, if there were no refractions, and about six minutes every day, at a mean rate.

INTERROGATIONS FOR SECTION SIXTH.

Of what does LIGHT consist?

What would be the consequence, if the particles of light were sufficiently large to be discovered by our best microscopes?

What is meant by the retina?

Is light reflected in all directions?

What is a ray of light?

How is it ascertained that rays of light move in direct lines?

Is light reflected from the Moon to our Earth?
Is light sent from one star to another ?

In what time does light pass from the Sun to the

Earth?

How many miles per second is its velocity?

Are the rays of light passing from any luminous body, interrupted by those from any other ?

Are the quantities of light received upon any given plane, diminished by being removed at a greater distance from that plane?

In what proportion are they diminished ? What appearance have the moons or planets, when their disks are magnified by the aid of a telescope? When a ray of light passes out of one medium into

another, is it turned out of its former course? If it does, how can it be proved?

With what is the earth surrounded ?

What is the nature of this fluid?

Is it capable of being compressed?

Is it more dense at the surface of the earth, than some distance above?

How much heavier is water than atmospheric air? What is the mean height of the atmosphere?

By what means is the weight of air at the surface of the earth found?

What is the weight of air on every square inch?
How many pounds on a square foot?

How many on the surface of the body of a common sized man?

In what kind of weather is the air lightest?

How is it proved?

In what kind the heaviest ?

At what height may the clouds be borne by the density

of the atmosphere?

How is air rarified?

By what is the Sun discovered, when he is in reality below the horizon?

SECTION SEVENTH.

TO FIND THE DIAMETER OF THE EARTH, AND THE DISTANCES OF THE SUN AND MOON, AND THE DISTANCES OF ALL THE PLANETS FROM THE SUN.*

It has been observed that a person at Sea, and exactly under the equator, discovers both the north and south polar stars, just rising in the horizon. Therefore as you advance towards either pole, the star will appear to rise higher in the horizon, and if you advance ten degrees from the equator towards the north pole, the polar star will there be ten degrees above the horizon, and consequently, if the angle of the elevation of that star be taken at any place, the number of degrees of its elevation, will be equal to the north latitude of the place where the elevation was taken. It has been ascertained by actual measurement, that one degree of the surface of the earth contains 69 and miles nearly. Then, as one degree is to 69 and miles, so is 360 degrees to the circumference of the earth. The diameter can then be found by the following proportion; as 355 is to the circumference, so is 113 to the diameter.

Let a large graduated instrument having a more able index, with sight holes, be prepared, in such manner, that its plane surface may be parallel to the plane of the equator, and its edge in the meridian, so that when the Moon is in the equinox, and on the meridian, she may be seen through the sight holes when the edge of the moveable index cuts the beginning of the divisions on the graduated limb, and when she is so seen, let the pre

• NOTE-Persons unacquainted with Trigonometry, may pass over this Section, as they will not be capable of forming correct ideas of the methods of finding the accurate distances, and therefore must take the Philosopher's word.

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