1 66 Professor Forbes's Experiments on 



stals, which dissolve in water and in alcohol,- but not in a?ther, 

 and the solutions of which are thrown down as a red precipi- 

 tate by iron and its salts. 



[To be continued.] 



X X. Account of some Experiments made in different Parts of 

 Europe, on Terrestrial Magnetic Intensity, particularly with 

 reference to the Effect of Height. By James D. Forbes, 

 Esq., F.R.SS. L. $■ E., fyc, Professor of Natural Philosophy 

 in the University of Edinburgh. 



[Continued from p. 66, and concluded.] 



18. V. Variations in the Needles' Magnetism. — In all obser- 

 vations of this kind, this change gives rise to the most trou- 

 blesome errors. The mode of ensuring an equable magnetic 

 state is unknown, though an approximation may generally be 

 obtained to it. Of the two needles sent to this country in 

 1827, by Professor Hansteen, one (No. 1.) has, after some 

 slight variations, become almost stationary in its magnetism ; 

 the other " Flat'' has been continually diminishing in intensity. 

 We have seen in the last article that the earth's magnetic ac- 

 tion, varying continually and being unknown, we can only 

 properly compare observations made at the same time of the 

 year, and of the day. The progress of change in the needles 

 may be traced by the following tables*. 



Table IV. 



* The mutual action of the needles is a point of importance. Before 

 they came into my possession they were kept in their separate cases, but 

 without further attention, being packed together in the same external case 



