Since Lalande’s day there has been more than one Comet “scare.” One of these startled Europe in 1832. On October 29th of that year, Biela’s Comet crossed the Earth’s orbit. The announcement was received with stupefaction. It was only when Arago soothingly pointed out that the Earth would not reach the exact point where the Comet had intersected the Earth’s orbit until November 30, at which time the Comet would be 50,000,000 miles away, that the popular excitement subsided. A similar alarm seized the world in 1857. Some prophet declared that on June 13 the world would collide with a certain periodic Comet having a period of revolution of three centuries. It is related that the churches and confessionals were crowded for days. Still another prediction, made in 1872 by Plantamour, the distinguished director of the Geneva Observatory, set Europe in a ferment. His calculations were based on errors, which were pointed out by other astronomers, and the public mind was quieted. Although more than two centuries have passed since Halley was in his prime, the possibility of a collision with some vagabond star still haunts the mind of the astronomer. That a collision is apt to occur is an admitted astronomic fact. The latest estimate, made in 1909 by Prof. William H. Pickering of Harvard University, would seem to prove that the core of one Comet in about 100,000,000 Comets will hit the earth squarely. An encounter with some part of a Comet’s head will happen once in 4,000,000 years. Since Comets’ orbit are more thickly distributed near the ecliptic than else where in the celestial sphere, the collisions will occur according to Pickering, perhaps more frequently than this. Because Pickering’s figures differ from those other astronomers—Arago and Babinet, for instance—it must not be inferred that his predecessors are wrong and that he is right in his calculations. The problem is too complex for that. Pickering, Arago and Babinet differ partly because they have assumed different average sizes for their Comets, and partly because their definitions of visible Comets are not in accord. That the possibility is very real, we shall all have an opportunity of judging on May 18, 1910. On that date the Earth will be plunged in the tail of Halley’s Comet, and the head will be less than 15,000,000 miles away—a mere hand’s breadth in the vastness of the universe. What will happen? Nobody knows for certain. By means of the wonderful instrument called the spectroscope, an instrument which analyzes a distant star as readily as if it were a stone picked up in the road, it has been discovered that a Comet’s tail is composed of gases called “hydrocarbons” (combinations of hydrogen and carbon), and that it bears a close chemical resemblance to the blue flame of a kitchen gas-stove.