92 COSMOS. 



that it remains entirely within the Tin- its of our planetary 

 system, attaining its aphelion between the orbits of the 

 smaller planets and that of Jupiter. Its eccentricity must 

 be assumed at 0*845, that of Juno (which has the greatest 

 eccentricity of any of the planets) being 0*255. Encke's 

 comet has several times, although with difficulty, been ob- 

 served by the naked eye, as in Europe in 1819, and, according 

 to Rumker, in New Holland in 1822. Its period of revolu- 

 tion is about 3i years ; but, from a careful comparison of the 

 epochs of its return to its perihelion, the remarkable fact has 

 oeen discovered, that these periods have diminished in the 

 most regular manner between the years 1786 and 1838, the 

 diminution amounting in the course of 52 years to about 

 I-^Q days. The attempt to bring into unison the results of 

 observation and calculation in the investigation of all the 

 planetary disturbances, with the view of explaining this phe- 

 nomenon, has led to the adoption of the very probable hy- 

 pothesis, that there exists, dispersed in space, a vaporous 

 substance capable of acting as a resisting medium. This 

 matter diminishes the tangential force, and with it the major 

 axis of the comet's orbit. The value of the constant of the 

 resistance appears to be somewhat different before and after 

 the perihelion; and this may, perhaps, be ascribed to the 

 altered form of the small nebulous star in the vicinity of the 

 Sun, and to the action of the unequal density of the strata of 

 cosmical ether.* These facts, and the investigations to which 

 they have led, belong to the most interesting results of 

 modern astronomy. Encke's comet has been the means of 

 leading astronomers to a more exact investigation of Jupiter's 

 mass (a most important point with reference to the calcula- 

 tion of perturbations) ; arid, more recently, the course of 

 this comet has obtained for us the first determination, al- 

 though only an approximative one, of a smaller mass for 

 Mercury. 



The discovery of Encke's comet, which had a period 01 

 only 3i years, was speedily followed, in 1826, by that of 

 another, Biela's comet, whose period of revolution is 6| 

 years, and which is likewise planetary, having its aphelion 

 beyond the orbit of. Jupiter but within that of Saturn. It 



*Encke, in the Astronomische Nachrichten, 1843, No. 489, a. 130-182 



