DISCOVERY BY MEANS OF CALCULATIONS. 609 



swaying motion of the axis, and exhibits to us continually 

 first a little of one side, and then of the other. 



The astronomer Encke, having, by calculation, pre- 

 dicted that his comet would return every 3 years, a 

 Frenchman named Pons, at Marseilles, observed it in 

 1819, and found that it arrived 2 hours earlier than the 

 calculated time. By calculating the effects of the diffe- 

 rent planets upon his comet, Encke also discovered that 

 Mercury is smaller, and Jupiter much larger, than previous 

 astronomers believed. By similar means Biela's comet 

 was discovered by Biela, an Austrian officer, in 1826 ; 

 Claussen computed its orbit, and found that the comet 

 appeared every 6 years and 8 months, and subsequent 

 re-appearances of it proved the correctness of the calcu- 

 lation. Its expected return in the year 1832 caused great 

 consternation, especially in France, because it would then 

 cross the earth's orbit, and people thought it would 

 destroy the earth; but the latter was in a distant part of 

 her orbit at the time. The alarm was so great in Paris, 

 that Arago, at the request of the French Academy of 

 Sciences, wrote a popular essay explaining the circum- 

 stances, in order to pacify the public. On November 26, 

 1845, it returned in accordance with the calculations, and 

 Lieutenant Maury then discovered by observation that it 

 had split into two portions, each a perfect comet. The 

 two pieces returned in company, at the same distance 

 apart, in the year 1852, but have never been seen again. 

 It was by working out mathematical calculations that 

 both Adams and Le Verrier, in 1846, were enabled to 

 affirm the existence of the undiscovered planet Neptune, 

 and whereabouts it might be found. It was also by 

 means of calculations that Schiaparelli, in the years 

 1862-3, discovered, and confirmed his suggestion, that a 

 comet which was seen in 1862 travelled along the same 



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