Page:The evolution of worlds - Lowell.djvu/58

34 Thus they argued that the meteors must all be travelling in parallel lines along an orbit which the previous shower, of 1799, showed to be periodic. This was the first recognition of a meteor-swarm.

The next advance was when Schiaparelli, in 1862, pointed out the remarkable connection between meteor-swarms and comets. On calculation the August meteor-stream and the comet of 1862 proved to be pursuing exactly the same path. Soon other instances of like association were discovered, and we now know mathematically that meteor-streams can be, deductively that they must be, and observationally that they are, disintegrated comets. More than one comet has even been seen to split.

Then came the recognition that comets are not visitors from space, as Sir Isaac Newton and Laplace supposed, but part and parcel of our own solar system. Without going into the history of the subject, which includes Gauss, Schiaparelli, and finally Fabry's great Memoir, much too little known, the proof can, I think, be made comprehensible without too much technique, thanks to the fact that the Sun is speeding through space at the rate of eleven miles a second.

Orbits described by bodies under the action of a central force are always conic sections, as Sir Isaac Newton proved. There are two classes of such curves: those which return into themselves, such as the circle and