201 
Mr Porrer said he remembered some brilliant displays not 
in a ten-year period, especially one of 1833; and he made some 
remarks in corroboration of Professor Challis’ statements, and 
objected to the “dust” theory. 
Mr Srvarr mentioned that the spectrum of the Aurora had 
been observed and shewn the same line between D and FL, as 
had the zodiacal light; he briefly replied to Mr Potter. 
Professor SELWYN exhibited some heliographs of the sun, 
extending over a period of 12 days, ending Sept. 30, all which 
had been very favourable for photography, at the time of the 
sun’s maximum. They shewed a very remarkable number of 
spots. Another series, taken from Aug. 22—29, shewed also a 
large outbreak of spots. He called attention to the fact that in 
the northern and southern hemispheres, tornadoes went as the 
deflections in the magnetic needles, mentioned by Mr Stuart, 
viz. in opposite directions ; and so did the sun’s spots. 
February 13, 1871. 
The PresIpENT (PRorEssoR CAYLEY) in the Chair. 
Communication to the Society : 
On the Operations of the Great Trigonometrical Survey 
of India in connexion with Geodesy. By Col. 
J.T. Wanker, RE, F.RS., Superintendent of 
the Survey. 
[ Abstract. | 
After pointing out that there are three stages in the ope- 
rations of a scientific national survey, namely, the trigonome- 
trical or geodetic basis, the topographical delineation of the 
ground, and the construction of the maps, Col. Walker observed 
that the first stage has frequently been ignored, and that during 
the last century this was the case in India, surveys being carried 
