COMETS OF SHORT PERIOD. 187 



into the toils, may be reckoned ' almost at naked 

 nothing.' 



We have been careful to dwell on this point for a 

 reason which will appear presently. 



The search for LexelFs comet led to the discovery of 

 a considerable number of nebulae ; and the discovery 

 of nebulae led in turn to the discovery of another comet 

 of small period. In 1786 Mechain announced to Mes- 

 sier (who had constructed a list of 103 nebulae) that he 

 had discovered a nebulous object. This turned out to 

 be a telescopic comet. It was again seen by Miss 

 Caroline Herschel in 1795, by Thulis in 1805, and by 

 Pons in 1818. All this time no suspicion had arisen 

 that these observers had seen the same object. But in 

 1818 the comet remained in view so long that it became 

 possible to calculate its orbit. This was done by the 

 Grerman mathematician Encke, who found that the 

 orbit is an ellipse, and the period of revolution about 

 three years and four months. He found, after a labori- 

 ous process of calculation, that it could be no other 

 than the object that attracted attention in 1786, 1795, 

 and 1805. Encke then applied himself to calculate 

 the next return of the comet, which he did so success- 

 fully that astronomers have continued to call by his 

 name the object whose motions he had been the first to 

 interpret. 



Encke's comet was seen by one observer only in 

 1822, as it was not favourably situated for observation 

 'in the northern hemisphere that observer was M. 

 Riimker, who followed the comet for three weeks at 



