Optics-'-^Magnetism* 169 



conclude, that the symptoms I have mentioned constitute a natural pro- 

 perty inherent in the insect, and produced when its light and numbers 

 are in a sufficient degree of intensity, we at the same time ascribe to it a 

 power which no luminous animal or other substance possesses but itself. 

 In conclusion, I have only to remark, that having made some experiments 

 on the specific gravity of the sea, I found that it decreased, with slight ex- 

 ceptions, gradually from the line, so that, if these are correct, the ani- 

 malcules are found smaller and more numerous where the specific gravity 

 of the sea is the least, provided at the same time the temperature admits 

 of their existence. 



MAGNETISM. 



6. Eaton's Proposed Improvement on Magnetic JVeedles.-^Viofessor 

 Amos Eaton proposes that the needles of compasses should be tipped with 

 silver, brass, &c. This not only preserves the points from rust, but with- 

 draws the poles from any attractive power in the brass, whether it arises 

 from hammering, or from any particle of steel or iron which may have 

 been accidentally left in the brass. — Dr Silliman's Journal, No. xxv. p. 16. 



7. Professor Hansteens Magnetic Tour to Siberia, — -We mentioned in 

 our last Number that this eminent philosopher proposed to set Out to Siberia, 

 to examine the intensity of the earth's magnetism in that vast country. 

 We are happy to inform our readers that the national assembly of Nor- 

 way has voted the necessary funds for this purpose. Dr Erman Junior, 

 the son of the celebrated Professor Erman of Berlin, has offered his ser- 

 vices as the companion and assistant of Professor Hansteen, and from his 

 knowledge of mineralogy and geology, we may expect much important in- 

 formation respecting Siberia. Professor Hansteen sets out in March, from 

 Christiania for Stockholm and St Petersburgh, where he joins his compa- 

 nion from Berlin. We trust that Professor Hansteen will endeavour to ob- 

 tain some information respecting the mean temperature of Siberia, which 

 acquires a great interest from the proximity of the Asiatic Pole of maxi- 

 mum co\d»— -Letter Jrom Professor Hansteen. 



8. On the Magnetic Actions excited in all Bodies by powerful Magnets* 

 —In a memoir on this subject read to the Academy of Sciences, on the 

 17th September, M. Becquerel points out the positions in relation to a 

 powerful magnet, assumed by a small cartouche of paper filled with tritox- 

 ide of iron, or a needle of any substance whatever freely suspended, and 

 the position of whose centre of suspension varies in relation to the nearest 

 pole of the needle. When this centre is very near one of the extremities, 

 and in a line parallel to the line of the poles, the needle will place itself in 

 a direction perpendicular to this line, in place of taking the direction 

 of the line which a magnetical needle would have done. If the centre of 

 suspension is brought nearer the magnet, from 1 to 5 millimetres, the 

 needle then deviates from the perpendicular direction. From numerous ex- 

 periments, M. Becquerel has shown that the essential difference in the 

 phenomena produced in a needle of iron, or steel, and needles of other 

 substances is, that in the first, the distribution of magnetism is constantly 



