62 Meetings of the Scientific Association of Great Britain. 
netic intensity presents scarcely any anomalies when observations 
are made on extinguished volcanoes; while the contrary is extremely 
sensible on volcanoes in action; from this it would seem that these 
anomalies depend upon chemical actions. 
I availed myself of my journey in England, to confirm the ob- 
servations on the relative intensity of magnetic forces at Paris, Lon- 
don, and Brussels, which Capt. Sabie was so good as to communi- 
cate to me. I believe that Prof. Forbes has repeated the same ob- 
servations on his part, which will give a valuable source of correc- 
tion. Mr. Snow Harris, intends to determine the difference of in-’ 
tensity between Cambridge, London, and Plymouth. . 
Mr. Snow Harris, (whose very ingenious magnetical and electrical 
apparatus I regret that I cannot here describe,) in order to ascertain 
the horizontal state of the needle, (for intensity,) suspends it over a 
liquid and endeavors to produce parallelism between the needle and 
its image. , 
The doubtful question was also discussed of the degree of pre- 
cision which may be obtained by magnetic instruments ; some of the 
gentlemen, for instance, doubted whether the dip could be obtained 
nearer than one fourth of a degree, others mentioned observers who 
thought’ they could obtain it within half a minute. It is not unin- 
teresting to know the limits of precision in the opinion of philoso- 
phers, as it gives us a standard of the actual condition of the science, 
and of the mechanic arts. Mr. Scoresby exhibited a variation com- 
pass, with a number of circles, designed to supersede, in many cases, 
astronomical instruments in determining the elements of situations at 
sea. Admiral Brisbane agreed with him as to the panei offer- - 
ed by this instrument. 
This naturally leads me to speak of the globe, which Mr. Barlow 
was so good as to show me at Woolwich, and upon which this phi- 
losopher had traced lines, showing the places which gave the same 
variation of the needle, according to the latest observations. ‘The 
coasts appeared to have a very marked influence on the deflections ; 
1 was struck the next day with seeing at Mr. Bailly’s that the result 
of the observations, on the pendulum left by Capt. Forster, which 
have just been calculated afford almost the same discrepancies. 
Mr. Bailly, who was so obliging as to show me Capt. Forster’s 
papers pointed out to me a very curious result, which is in fact con- 
firmed by very few observations, but which, on that very account, 
deserves to be further verified ; it is that the oscillations of a pendu- 
