A418 Scientific Intelligence. 
Our trials were nd confined to inanimate objects, but good portraits 
of my friends and assistants, Mr. Brush a 
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almost any cay. But in the dark and murky atmosphere of London, 
it may become an important auxiliary in the art. Considering the easy 
manipulation of a good carbon pile, its constancy and the facile man- 
agement of the light with the ingenious but simple apparatus of Duboseq, 
it certairly offers a tempting field of investigation as well to the practi- 
cal dagierreotypist as to the student in optics. It is much more intense 
and equally manageable with the lime light, and consequently may be 
substiuted for it in all optical demonstrations. Whether the properties 
of galvanic light are in all respects similar to solar light remains also 
an interesting question for future investigation. 
Laboratory of Louisville University, February, 1851. 
3. Attraction of Electro-magnets.——Dvs has published the results 
of an elaborate investigation of the attractive forces exerted by elec- 
tro-magnets upon armatures of different dimensions and placed at dif- 
ferent distances from the magnet. The method of measurement em- 
loyed was similar to that made use of by the author in a previous 
investigation, and consisied in observing in each case the weight 
necessary to tear the armature from the magnet itself, when the two 
were in actual contact, or to hold the attracting force in statical 
equilibrium when this was not the case. ‘The magnets employed were 
straight cylindrical bars; the armatures similar bars of various dimen- 
" sions attached successively to the arms of a balance, and held in exact 
equilibrium by weights placed in the opposite scale pan; the appa- 
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: e diminution of the attraction is greater in proportion as the 
attraction itself is small when the attracting surfaces are in contact; 
the surface of contact remaining the same. a 
o give a definite idea of the absolute amount of this diminution, 
the author states that in no one of his experiments was there an attrac- 
ting force which at a distance of +3, of an inch was greater than half 
or less than one-fourth of the attraction during contact. = 
(3.) After a certain distance, the attraction is less in proportion as | 
magnet or the armature or both are thinner. (Although in genera 
attraction is not directly proportional to the diameter of the 
armature.) ; : eer: 
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