6 
Playfair, in support of his arguments when discoursing of the 
value of abstract science ; but as it will be my duty to show you 
in the progress of my course the experimental evidence to which 
I have now referred, I have not hesitated about repeating the 
striking illustration which these physical forces lend to the posi- 
tion I maintain. I cannot allow the present opportunity to 
pass without drawing attention to the evidence afforded by the 
attempts which have been made to apply the electrical element 
as an illuminating and mechanical power, of the danger of at- 
tempting to do so before we have acquired an accurate know- 
ledge of all the phenomena which are involved in the conditions 
of the application. In the arc of light produced between the 
poles of a powerful voltaic battery we obtain the most vivid 
illumination which can be produced by artificial means. In- 
genious mechanical contrivances have been devised to render 
the distance uniform between the charcoal points, which form 
the polar terminations of the wires, and thus to give steadiness 
to the electrical light, but since there is a constant transfer of the 
solid element from one pole to the other, the required steadiness 
has not yet been obtained. The question of the economy of this 
application, even if successful, may be conveniently embraced in 
connexion with the consideration of the application of electro- 
magnetism as a motive power. It has been stated, and is 
indeed now well known, that we can obtain an enormous 
attracting and repelling force by the agency of electro-magnets. 
In some cases magnets have been placed upon the periphery of 
a wheel, while others have been fixed upon a firm circle sur- 
rounding it. By mechanical contrivances the polar disposition 
of the magnets has been rapidly changed. The consequence of 
this is the exercise ol an attracting and repelling power, by 
which the wheel is driven swiftly round. Another form of 
construction, offering some advantages, is a cylinder, around 
which clothed copper wire has been coiled, thus forming a 
hollow magnet, into which another solid magnet represent- 
ing a piston is drawn, and from which, by changing the 
poles, it is afterwards repelled. By this means a long stroke 
is obtained, and by a crank any kind of machinery set 
motion. 
