COMETS. 23T 



through which the Hght of the sun can penetrate, and be seen on the side oppo- 

 site to that which it first strikes.. The coma and tail must be a very rare or thin 

 substance, as the Hght from a sixth magnitude star is sufficient to penetrate and 

 pass through it, where it is more than a milfion miles in extent ; the star's position 

 being the same as before, proving the density to be insufficient even to refract 

 light. 



Comets have, at different times passed very near the planets but have never 

 been found to cause any perturbations or irregularities in their orbits, while their 

 own were materially affected. Like the planets they move round the sun in conic 

 sections, their orbits are in the form of an ellips.e, parabola, or hyperbola. If an 

 ellipse, the eccentricity is usually very great ; those that move in this form of 

 orbit return after a series of years ; but those that move in parabolas, or hyper- 

 bolas, never return, but after wheeling about the sun once, will continue to re- 

 cede into infinite space for ever, and may visit other systems and suns. When, 

 near the sun they move with great velocity, sometimes at the rate of twenty-four 

 million miles a day. It is the observed velocity relative to its distance from the 

 sun that enables astronomers to determine the form of the orbit ; if at the distance 

 of the earth its velocity is equal to 2,400,000 miles a day, it will move in a para- 

 bola, if it exeeds that velocity in a hyperbola, if less, in an ellipse. 



In many cases the velocity is so near the rate stated that it is impossible to 

 decide whether they will return or not, it is however, certain that their return 

 will be delayed many centuries, and, perhaps, thousands of years. 



The exact number of comets cannot be determined; over seven hundred 

 have been observed and recorded. Many have doubtless visited our sun without 

 being seen, reaching their perihelion in the day-time, or during cloudy weather. 

 Arago calculates the number that have appeared, and will appear within the orbit 

 of Uranus at 7,000,000; the same calculation extended to the orbit of Neptune 

 would increase the number four-fold. In the opinion of Kepler, the celestial 

 spaces are as full of them as the sea is of fish, only a small portion of them being, 

 within the range of telescopic vision. 



Among the remarkable ones that have appeared may be mentioned Halley's,. 

 which was first observed on August 19, 1682, and remained visible about one month 

 and disappeared. After Halley computed the elements of its orbit, he found by 

 comparison that it must be the same one observed by Kepler in 1607, and that 

 it moved in a very elliptic orbit, returning to the sun once in about seventy-six 

 years. This comet agaiii appeared and passed its perihelion March 12, 1759; by 

 adding seventy six years we find it should appear in 1835, and so it did, being 

 first observed on August 5th ; it approached the sun and passed its perihelion on 

 November i6th at 11 o'clock a. m., only three days after the time predicted by 

 De Pontecoulant, a French mathematician. It was followed until May 17, 1836, 

 when it disappeared from view aided by the most powerful telescopes; but an 

 astronomer can follow its course with the eye of science, which is almost as certain 

 as ocular demonstration. 



