COMETS. 



about 1200 days. This calculation was verified by the fact of its 

 return in 1822, since which time the comet has gone by the name 

 of Enckes comet, and returned regularly. 



It may be asked, How it could have happened that a comet 

 which made its revolution in a period so short as three years and 

 a quarter, should not have been observed until so recent an epoch 

 as 1818 ? This is expfained by the fact that the comet is so small, 

 and its light so feeble even when in the most favourable position, 

 that it can only be seen with the aid of the telescope, and not 

 even with this except under certain conditions which are not ful- 

 iilled on the occasion of every perihelion passage. Nevertheless, 

 the comet was observed on three former occasions, and the general 

 elements of its path recorded, although its elliptic, and con- 

 sequently periodic character, was not recognised. 



On comparing, however, the elements then observed with those 

 of the comet now ascertained, no doubt can be entertained of their 

 identity. 



20. In the following table are given the elements of the orbit 

 of this comet, as computed from the observations made upon it at 

 each of its three appearances in 1786, 1795, and 1805, before its 

 periodic character was discovered, and at its eleven subsequent 

 appearances up to 1852. 



TABLE I. 



Elements of the Orbit of Encke's Comet to 1852. 



The motion of this comet is direct ; and its period in 1852 was 

 3-29616 years, which is subject to a slight variation. 

 It is evident that between 1786 and 1795 there were two, 

 153 



