12 ASTRONOMY. 



referred to (which could never exceed six signs of 

 longitude) must become almost insensible in their 

 annual revolutions, and unimportant until a great 

 number of years intervene, either before or after the 

 invention. 



Density of Comets, or the reverse. 



The small eccentricity of the planetary orbits, and 

 the motion of the planets in the same direction, are 

 essential to the stability of the system, yet the co- 

 mets, which obey neither of these laws, do not affect 

 that stability. Some disturbance would no doubt 

 be caused by the comets that pass through our sys- 

 tem were they bodies of great mass, or contained a 

 great quantity of matter ; but there are many reasons 

 for supposing them to have very little density, so 

 that their power to produce any deviation in the 

 planetary orbits is wholly inconsiderable. 



On the 9th Nov. 1795, Sir William Herschel saw 

 a double star of the 12th or 13th magnitude, through 

 the middle of a comet, with very little diminution of 

 its brightness.* 



The nucleus of a comet has so little density, that 

 even when in proximity to a planet, it exercises no 

 appreciable attraction on it. The comet of 1770 ap- 

 proached the earth to a distance of only seven times 

 greater than that of the moon, without producing any 

 sensible action on it. 



Satellites of Jupiter. 



Although the satellites of Jupiter have only been 

 known since the invention of the telescope, about two 

 centuries ago, yet within that period, the quickness 

 of their revolutions has exhibited all the changes 



* Every observation made on comets tends to strengthen the 

 suspicion, that so far from being burning bodies, they are masses 

 of transparent fluid having very little density. 



