158 RECORD OF SCIENCE FOR 1887 AND 1888. 



Dr. Wilsiiig' has found the rotation period from observations of faculjB 

 to bo 25. U3, and Ijus detected no variation in velocity depending u[)on 

 the latitude. The difficulty of identifying faculre on their reappearance, 

 and ol measuring their positions with exactness, makes the result some- 

 what doubtful. 



Diameter of the sun. — Dr. Auwers has made a very exhaustive dis- 

 cussion of the sun's horizontal and vertical diameters from the meridian 

 observations of Greenwich, Washington, Oxford and Neuchatel with 

 special reference to the alleged variations in the mean annual diameters 

 following the period of the suuspot cycle. He concludes that there is 

 no valid reason for suppo.^ing the sun's diameter to vary, and that the 

 api)areut changes arise from insufiBciently determined personal equa- 

 tions, lie also points out that meridian observations are quite unsuited 

 for the determination of any possible elliptioity in the sun's disk, and 

 that there is no reason to conclude from these results that such ellip- 

 ticily exists. The several mean values of the sun's (assumed circular) 

 diameter are : 



Greenwich 32' 2" .'M:^. Oxford 32' 3". 19. 



Wasbiugtoii 32' 2". 51. Ncncliritel 32' 3" .27. 



The discordances are ascribed to instrumental or uneliminated per- 

 sonal peculiarities. 



In a second paper Dr. Auwers discusses the apparent changes of both 

 the horizontal and vertical diameter during the course of a year, de- 

 duced from meridian observations, and he concludes that the periodic 

 variations in the monthly value of the diameters result not from 

 physical changes in the sun, but from the efiect of temperature on the 

 instruments and from difference in the quality of the telescoi)ic images 

 at opposite seasons of the year. 



Another discussion of the horizontal diameter of the sun luvs been 

 made by Professor di Legge from meridian transits of the sun observed 

 at Campidoglio from 1874 to 1883. The mean horizontal diameter at 

 mean distance deduced from 570G transits by four observer's on 2213 

 days is 32' 2".38. 



Solar activity in 1887, 1888. — The decrease in spots, facuhi? and promi- 

 nences which was so marked during 188G, and particularly during the 

 latter part of that year, continued in 1887, and although there was no 

 spotless period as long as that of November, 188(5, the mean si)otted 

 area for the year was much below that of the year preceding. The days 

 of greatest spotted area were July G, 7, and 8. The agreement in the 

 general form of the curves for spot numbers and magnetic variation 

 was not so close as in some previous years. The tluctuations in the num- 

 bers and dimensions of the promineiiees were less than for the spots, 

 but they also showed a maximum in July. Facuhe accorded well with 

 the prominences, neither facuhv nor prominences following the spots in 

 the marked depression of November. 



