ASTRONOMY. 117 



east side. It is not often, however, that this hap- 

 pens, as the spots dissolve and perish, from causes 

 that are to us quite unknown. 



b. The spots of the sun were first discovered by GA- 

 LILEO at Florence in 1611. Istoria e Demonstra- 

 zione intomo alle Macchie Solari. Opere di GA- 

 LILEO, torn, ii. p. 85., &c. Edit, di Padova, 1744. 



115. The position of a spot, relatively to the 

 sun's centre, may be determined by observing 

 the difference between the time when the cen- 

 tre of the sun passes the vertical wire of a tran- 

 sit telescope, and when the spot does the same. 

 This gives the difference in right ascension ; 

 and the difference in declination may be deter- 

 mined at the same time, by means of the move- 

 able wire in the telescope. In this way, by ob- 

 servations made day' after day, the path of the 

 spot on the sun's disk maybe traced with great 

 exactness. 



a. When the spots are followed in this way, many 

 of them are found to change, and to disappear al- 

 together in the course of a few days. Sometimes 

 a number of small spots unite into one large spot ; 

 at other times, a large spot separates in a number 

 of small ones, which soon disappear entirely. A 

 few of them, more permanent than the rest, may 

 be followed by the observation here described, as 

 long as two or three revolutions. 



116. The 



