groups might be expected to give the sun's distance without any uncertainty greater than the thousandth part of the total amount. The chief difficulty of the process arises from the movement of the planet during the interval which divides the evening from the morning observations. This drawback can be avoided by diligent and repeated measurements of the place of the planet with respect to the stars among which it passes. In the monumental piece of work which issued in 1897 from the Cape Observatory, under the direction of Dr. Gill, the final results from the observations of Iris, Victoria, and Sappho have been obtained. From this it appears that the angle which the earth's equatorial radius subtends at the centre of the sun when at its mean distance has the value 8".802. If we employ the best value of the earth's equatorial radius we obtain 92,870,000 miles as the mean distance of the centre of the sun from the centre of the earth. This is probably the most accurate determination of the scale of the solar system which has yet been made. CHAPTER XII. JUPITER. The Great Size of Jupiter--Comparison of his Diameter with that of the Earth--Dimensions of the Planet and his Orbit--His Rotation--Comparison of his Weight and Bulk with that of the Earth--Relative Lightness of Jupiter--How Explained--Jupiter still probably in a Heated Condition--The Belts on Jupiter--Spots on his Surface--Time of Rotation of different Spots various--Storms on Jupiter--Jupiter not Incandescent--The Satellites--Their Discovery--Telescopic Appearance--Their Orbits--The Eclipses and Occultations--A Satellite in Transit--The Velocity of Light Discovered--How is this Velocity to be Measured Experimentally?--Determination of the Sun's Distance by the Eclipses of Jupiter's Satellites--Jupiter's Satellites demonstrating the Copernican System.