MEMOIR OF ENCKE. 197 



or, in more general terms, that of a yet unknown cause operating upon both 

 comets, still remains undecided. 



Two other important investigations of Encke related to the sun's parallax, as 

 derived from the transits of Venus in 1761 and 17G9, the last of which Cook 

 Lad observed at Tahiti. After coiTCction of the observations, revised by 

 Father Hell in Vieinia,. Encke found the parallax to be 8.57 seconds. ''You 

 have turned to account," wTotc Bessel, '' what had l)een collected by the cxjxmi- 

 diture of vast sums and by the efforts of many, and thus those efforts have first 

 achieved success and still point onward to a step in advance." This magnitude 

 of the parallax was for a long time generally accepted; but subsequently, from 

 the opposition of Mars and the disturbances of the moon, was estimated at 8.9 

 seconds. The difference between this value and that found by Encke arose 

 from the circumstance that the position of the places of observation was not 

 ascertained with the requisite precision. After a more accurate determination of 

 this point in later times, the sun's parallax, as deduced from those old observa- 

 tions, has been shown by Powalky to amount to only 8.832 seconds.* 



Encke's personal position had so far changed, soon after his accession to the 

 observatory, that the charge of its management had devolved on him, at first 

 partially, through the intermitting attendance of Lindenau, and soon wholly, 

 from the absorption of the latter in administrative afthirs. As no other duties 

 claimed his time, Encke could now resign himself to scientific labor with free and 

 entire devotion. The value which he placed upon this rare immunity is seen 

 from a letter in which he declined an appointment tendered him of a professor- 

 ship in the University of Greifswald. "i'he chair of mathematics and astnuiomy 

 Lad there become vacant by the death of Professor Droysen, in 1814, and for 

 some years no successor was ajjpointcd. Bessel called Encke's attention to this, 

 and as, in regard to external circumstances, the situation presented great advan- 

 tages over that of Seeberg, proposed to recommend him for it. The philosoph- 

 ical faculty at the same time consulted both Gauss and Bessel in regard to a 

 choice. The preference of Gauss fell upon Gerling, who had already approved 

 himself as a teacLcr, tliougli next lie recommended Encke, wLom, in regard to 

 knowledge and capacity, lie rated quite as Lighly. Bessel, on the other hand, 

 named Encke in the first place : " He is a young man whose character is as 

 amiable as his acquirements in astronomy and mathematics are distinguished ; 

 nor is his skill as a practical astronomer less so. Many are the admirable labors 

 for which we are already indebted to Lim, evincing a diligence and conscienti- 

 ousness lieyond praise." 



To the question now addressed by the faculty to Encke, whether he would 

 permit himself to be placed in nominati(Ui for the office, he replied, INlarch 16, 

 1818, as follows: "My place here as adjunct of the observatory I have now 

 occupied for two years, and must, in justice, regard it as eminently fortunate, 

 seeing Low much my student years were abridged by tlio distractions of the war, 

 that I Lave been assigned to a position so wholly free from the demands of 

 other business, provided with so excellent a stock of instruments, under a direc- 

 tor (Herr Von Lindenau) whom Germany honors as one of its first astronomers, 

 and where, in short, it is permitted me to live exclusivel}'' for science. This i)Osi- 

 tion has at present so far changed that the director, involved in many other affairs, 

 and but just returned after a year's absence. Las no prospect but to be again and 

 repeatedly absent in the course of the cun-ent year. Under these circumstances 

 my emplo3'ers have taken occasion so clearly to indicate their wish that I should 

 remain here, tLat I sLould little respond to their jjrevious kindness were I not 

 ready to forgo the great advantages proffered me by tLe Greifswald University. 



* [This element has been recently iiivestif^atedat the Naval Obsei-vatory, Washiugton, by 

 Professor Nevvconib, from all. the more valuable data of recent times, and its value is thus 

 fixed atC)".C!5, with a probable error of not more thau two or three hundredths of a second. 



J. II.] 



