OP ARTS AND SCIENCES. 



37 



Adopting the curve described above as representing the true varia- 

 tion, its ordinates for every ten years have been read off, and are given 

 in the third column of Table XIII. The direction of its tangent has 

 also been determined, and the seconds of the resulting period is en- 

 tered in the fourth column. To this is to be added 2*^ 20''- 48'"- 

 The second column gives approximately the corresponding value of E. 



TABLE XIII. — Variation of Period. 



An inspection of the curve of variation of the times of minimum 

 shows that a curious change took place between 1830 and 1850. Be- 

 fore then, the period given by Wurm of 2^ 20^- 48™ 58.5' repre- 

 sents the observations well; after 1850, the formula of Schonfeld 

 appears to be more nearly correct. There seems, during this interval, 

 to have been a change of four or five seconds in the period, and that 

 besides this there has been a small but gradually increasing diminution 

 in the period throughout the century. 



Harvard College Observatokt, 

 Cambridge, U. S. 



