282 SPECIAL RESULTS IN THE URANOLOGICAL 



variability of their distances from other spots visible at the 

 same time, afford an apparently satisfactory degree of se- 

 curity. 



Although solar spots may much oftener than is generally 

 supposed be distinctly recognised by the unassisted eye of 

 an observer looking for them, yet after careful investigation 

 we find, between the beginning of the 9th and of the 17th 

 centuries, at the utmost, not more than two or three notices 

 of their appearance upon which we can depend. I reckon as 

 such the supposed presence of Mercury upon the sun's disk 

 for a period of eight days, in the year 807, recorded in the 

 annals of the kings of the Franks, which were ascribed 

 first to an astronomer belonging to the Benedictine order, 

 and afterwards to Eginhard ; the transit of Venus over the 

 Sun, lasting 91 days, said to be observed under the Caliph 

 Al Motassem in 840 ; and the " Signa in Sole" in the year 

 1096, according to the Staindelii Chronicon. The his- 

 torical records of occasions on which the Sun has been 

 darkened, or, as it would be more accurately expressed, 

 when there has been during a longer or shorter time a 

 diminution of the light of day, have induced me for a 

 long time past to institute particular inquiries into such 

 meteorological, or possibly cosmical, phenomena ( 481 ). As 

 extensive series of solar spots (those observed by Hevelius 

 on the 2()th of July, 1643, covered a third part of the Sun's 

 disk) are always accompanied by numerous faculw, I am 

 but little inclined to ascribe to their occurrence obscurations 

 during which stars were sometimes visible as in total 

 eclipses of the Sun. 



The diminutions of daylight related by annalists may, I 

 think, be classed under three heads according to three wholly 



