The variation in the orbital velocity of some comets is enormous. The velocity of the comet of 1680 when passing round the sun (perihelion) was about 212 miles a second! Whereas at its greatest distance from the sun (aphelion) the velocity is reduced to about 10 feet a second! CHAPTER XII Meteors Mr. Denning thinks that the meteor shower of the month of May, known as the Aquarids, is probably connected with Halley's comet. The meteors should be looked for after 1 a.m. during the first week in May, and may possibly show an enhanced display in May, 1910, when Halley's comet will be near the sun and earth.[231] On November 29, 1905, Sir David Gill observed a fireball with an apparent diameter equal to that of the moon, which remained visible for 5 minutes and disappeared in a hazy sky. Observed from another place, Mr. Fuller found that the meteor was visible 2 hours later! Sir David Gill stated that he does not know of any similar phenomenon.[232] Mr. Denning finds that swiftly moving meteors become visible at a greater height above the earth's surface than the slower ones. Thus, for the Leonids and Perseids, which are both swift, it has been found that the Leonids appear at an average height of 84 miles, and disappear at a height of 56 miles; and the Perseids at 80 and 54 miles respectively. "On the other hand, the mean height of the very slow meteors average about 65 miles at the beginning and 38 miles at the end of their appearance."[233] During the night of July 21-22, 1896, Mr. William Brooks, the well-known astronomer, and director of the Smith Observatory at Geneva (New York), saw a round dark body pass slowly across the moon's bright disc, the moon being nearly full at the time. The apparent diameter of the object was about one minute of arc, and the duration of the transit 3 or 4 seconds, the direction of motion being from east to west. On August 22 of the same year, Mr. Gathman (an American observer) saw a meteor crossing the _sun's_ disc, the transit lasting about 8 seconds.[234] A meteor which appeared in Italy on July 7, 1892, was shown by Prof. von Niessl to have had an _ascending_ path towards the latter end of its course! The length of its path was computed to be 683 miles. When first seen, its height above the earth was about 42 miles, and when it disappeared its height had increased to about 98 miles, showing that its motion was directed upwards![235] In the case of the fall of meteoric stones, which occasionally occur, it has sometimes been noticed that the sound caused by the explosion of the meteorite, or its passage through the air, is heard before the meteorite is seen to fall. This has been explained by the fact that owing to the resistance of the air to a body moving at first with a high velocity its speed is so reduced that it strikes the earth with a velocity less than that of sound. Hence the sound reaches the earth before the body strikes the ground.[236]