190 LIGHT SCIENCE FOR LEISURE HOURS. 



the earth would continue to approach the sun with 

 continually increasing velocity. 



Returning to Encke s comet, we have to notice yet 

 another important discovery which was effected by its 

 means. The comet passed so near to Mercury in 1835 

 as to enable astronomers to form a much more satis 

 factory estimate of this planet s mass than had hitherto 

 been obtained. It was found that the mass of Mercury 

 had been largely over-estimated. 



No very long interval passed after the discovery of 

 Encke s comet before another comet of short period 

 was detected. M. Pons, who had discovered Encke s 

 comet, it will be remembered, in 1818, observed a faint 

 nebulous object on June 12, 1819. This object turned 

 out to be a comet ; and in this case, as in the former, 

 Encke calculated the stranger s orbit and period. He 

 found that it moves in an ellipse which extends slightly 

 beyond the orbit of Jupiter, and that it has a period of 

 about five and a half years. This object was not seen 

 again, however, until the year 1858, when M. Winnecke 

 discovered it, and at first supposed it to be a new comet. 

 Calculation soon showed the identity of the two objects, 

 and confirmed the results which had been obtained by 

 Encke in 1819. 



The next comet of short period was discovered by 

 M. Biela in 1826. Perhaps nothing in the whole his 

 tory of cometic observation is more surprising than 

 what has been recorded of this singular object. We 

 must premise that the comet had been seen in March 

 1772, and again in November 1805. But it was not 



