OF ARTS AND SCIENCES. 



277 



A minimum is clearly iiiflicated in 1797, May 22, and the star 

 seems to have been below its maximum briglitness on the other nights 

 also. Converting the magnitudes into grades, and comparing with the 

 li"-ht curve of Professor Schonfeld, we may infer that on 1795, May 11, 

 the observation vras made 4'\7 before, or 5^.2 after, a minimum. The 

 observations of 1795, May 18, in like manner, indicate that a mini- 

 mum vrould follow in 3'\4, or had passed 3'*. 8. The observation of 

 1797, May 22, indicates a brightness that does not differ sensibly from 

 that at minimum. The star changes in brightness by about a tenth of 

 a magnitude within an hour of minimum. 



The hours within which the observations must have been made are 

 limited by the twilight, which would, for observations of such faint 

 stars, be appreciable within two hours of midnight. Tlie comparison 

 stars 43 and 44 Librce would be above the horizon from 8''.0 to 1G'\4. 

 Their altitudes would exceed 10° from 9''.2 to 15'\2. 



The period of S Librce is 2'^ 1^ 51"' 20» = 2-1.3273148. Ac- 

 cordingly, if a minimum occurred near midnight on 1797, May 22, 

 others would have occurred on the afternoons of 1795, May 11, and 

 1795, May 18. We may therefore assume that tlie observations of 

 1795 were made after, and not before minima. vSnbtracting from the 

 time 1795, INIay ir> 12*^, the interval 5''.2, adding 0'\2 for the differ- 

 ence in longitude of Slough and Paris, and adding 0*^.1 for the equa- 

 tion of light, we obtain 1795, May IT' 7^.1, for the Paris heliocentric 

 time of minimum. The times of minima indicated for the other ob- 

 servations are given with this in the first column of Table VI. The 

 second column gives the ephemeris time for the epoch given in the 

 third column. The last column gives the observed minus the com- 

 puted times of minima. 



TABLE VI. 



The mean of these results indicates a correction of six hours to the 

 ephemeris, or of seven hours, if we as.sign somewhat greater weight to 

 the last observation. A most fortunate coincidence broiitrjit all (he 

 observations so near minimum that the star had in eacli case less than 



