Pagina:Scientia - Vol. VII.djvu/250

242 scientia

paramount, and to which we may assign an arbitrary radius, such as 1000 times the radius of Neptune’s orbit) with zero relative velocity; in other words, it was previously moving through space with exactly the same motion as the translational drift of the solar system. The chances are slight of this being the case in a single instance, and it is out of the question that any considerable number of the comets whose orbits are sensibly parabolic should have reached our system from outside. The relative velocities of the stars are of the order of many kilometres per second, which would suffice to produce an orbit of most markedly hyperbolic character in a comet reaching us from another system.

It is then clear that it is to our own system that we must look for the origin of the comets with apparently parabolic paths, and still more those of elliptical character. Within this system there are three different modes of origin that have been suggested: 1) that they are the products of eruptions from the sun; 2) that they are the products of eruptions from the larger planets in a sunlike state; 3) that they are stray fragments of the nebula, which is supposed to have been the parent of our system, and that they remained unattached to any of the large masses that were formed from that nebula.

There are two points that are in favour of the solar origin; first, we can see, in the solar prominences, eruptions of gas at sufficient velocities to carry some of the projected matter away from the sun. Secondly comets by their spectrum, and meteors by actual analysis, reveal the presence of large amounts of hydrogen and its compounds, suggesting their origin in an atmosphere like that of the sun. The obvious drawback to this theory is that all matter ejected by the sun would travel in orbits intersecting his globe, and so, if their speed of ejection was less than 383 miles per second (the parabolic velocity) they would on their return fall back on the sun. Planetary perturbations might suffice to avert this, and produce an orbit just clearing his surface. We have instances of such orbits in the remarkable group of comets of 1680, 1843, 1880, 1882, 1887, and a solar origin does not seem impossible for these; but there are many other comets whose perihelion distance equals or even greatly exceeds the earth’s distance from the sun, and we can hardly