DISTANCES OF THE PLANETS FROM THE SUN.-The vast extent of the solar system is but vaguely to be conceived from the ordinary mode of stating it in millions of miles. To demonstrate it in a more striking and impressive manner, a continential astronomer has proposed, or rather renewed the proposal, that the computed distances of the planets be measured by comparison with the velocity of a cannon ball, rated at 1 1-2 German miles per minute. With this velocity a cannon ball fired from the sun, would reach the planet Mercury in 9 years and 6 months; Venus in 18 years; the earth in 25 years; Mars in 38; Jupiter in 130; Saturn in 238; and Uranus (Herschel) in 479 years. With the same velocity a shot would reach the moon from the earth in 23 days, little more than three weeks.