212 Mary Somerville. 



We were deprived of the society of Sir John and 

 Lady Herschel for four years, because Sir John took 

 his telescope and other instruments to the Cape of 

 Good Hope, where he went, accompanied by his 

 family, for the purpose of observing the celestial 

 phenomena of the southern hemisphere. There are 

 more than 6,000 double stars in the northern hemis- 

 phere, in a large proportion of which the angle of 

 position and distance between the two stars have 

 been measured, and Sir John determined, in the 

 same manner, 1081 in the southern hemisphere, and 

 I believe many additions have been made to them 

 since that time. In many of these one star revolves 

 rapidly round the other. The elliptical orbits and 

 periodical times of sixteen or seventeen of these 

 stellar systems have been determined. In Gamma 

 Virginis the two stars are nearly of the same magni- 

 tude, and were so far apart in the middle of the 

 last century that they were considered to be quite 

 independent of each other. Since then they have 

 been gradually approaching one another, till, in 

 March, 1836, I had a letter from Admiral Smyth, 

 informing me that he had seen one of the stars 

 eclipse the other, from his observatory at Bedford. 



