TABLES OF THE DIAMETERS, DISTANCES, &c. of THE PLANETS. Accompanied with various comparisons, in order to render the ideas of thefe distances, &c. clearer to the mind. When you endeavour to form any idea of diftance, magnitude, or duration, by numbers only, you foon exceed the limits of conception, and find your faculties of reasoning as finite as your fenfes. Hence aftronomers are frequently obliged to have recourfe to mixed ideas, and make things of different nature and properties affift each other, to excite more adequate ideas of what they would have expreffed. Some of these methods I fhall now lay before you, to affift your imagination in forming it's ideas of the vaft diftances and fizes of the planets, 40 LECTURES ON NATURAL PHILOSOPHY. MEAN DISTANCES IN MILLIONS OF MILES. The distances being very great, the neareft million only is inferted, that it may be eafier remembered. The following table from Mr. Vince's plan of a Courfe of Lectures, may be confidered as more accurate; it is deduced from M. de la Lande's work. The rotation of Saturn, agreeable to Dr. Usher's computations, is 10 hours, 12 minutes. A different refult was, however, obtained, by taking the density of Saturn, as ftated by M. de la Lande. Dr. Herschel has fettled the rotation of Saturn's' ring at 10 hours, 32 minutes, 16 feconds. Sun's apparent diameter, I min. 41 fec. DISTANCES AND APPARENT DIAMETERS OF THE SUN AND PLANETS. It has been found, that a cannon-ball moves about 8 miles in one minute, 704 feet in a fecond; and that found moves about 13 miles in one minute, 1144 feet in a fecond. A very high wind may make found move one mile in 4 feconds, 4 dee, that is, in about onetwentieth less time than in calm weather. The most violent ftorm does not move above I mile in a minute, 88 feet in a second. From hence it has been computed, that a body iffuing from the fun, with the swiftness of a cannonball, that is, 8 miles in a minute, would employ the following time in reaching. A ray of light comes from the fun to the earth in 8 min. 13 fec. moves therefore 11,693,462 miles in one minute, 194,891 miles in one fecond. A ray of light comes from the moon in i fecond, 23 dee. From the very accurate obfervations of Dr. Bradley, it is inferred, that no fixed ftar of the great numbers obferved by him, can be at a lefs diftance from the earth than about 400,000 diftances of the fun from the earth; fo that a ray of light, which comes to the earth in eight minutes, thirteen feconds, iffuing from fuch a ftar, muft rcquire 6 years and 3 months to reach the earth. The 1 1 |