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 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 formcr course? If it does, how can it be proved? 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? 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 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. 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 precise time be noted. As the Moon revolves about the earth from any meridian to the same again in 24 hours and 48 minutes, she will go a fourth part round it in a fourth part of that time; namely six hours and 12 minutes as seen from the earth's centre or pole: But, as seen from the observer's place on the earth's surface, the Moon will seem to have gone a quarter round the earth, when she comes to the sensible horizon; for the index through the sights of which she is then viewed, will be 90 degrees from where it was when she was first seen. Let the exact moment when she is in, or near the sensible horizon be carefully noted,* that it may be known in what time she has * Here proper allowances must be made, (for the refraction being about 33 minutes of a degree in the (horizon, will cause the Moon's centre to appear 33 minutes above the horizon. when her centre is really in it. |
