wn changing the Dimensions of Iron. 177 
of that of the rod] encloses the extremity of the bar; it 
diminishes as the helix approaches the point [the center] where 
the rod is clamped, and it is probable that when it is quite close 
to this point, the elongation changes into a retraction, but I 
have never been able to observe the motion in this direction 
* Hew I 
that it was not possible for me to measure this longitudinal 
traction ; happily Mr. Joule has supplied that omission. 
of bismuth, as it is known to do in the case of iron. e 
action, if any, was sure to be infinitessimal, and I therefore cast 
about for a means of magnifying it. ae ai con- 
sulted Mr. Becker, and thanks to his great intelligence and 
refined skill, I became the possessor of the apparatus now to 
bed. % * * * 
complimentary effects might be here exhibited, and a new 
antithesis thus established between magnetism and diamag- 
etism.” 
these rods, securely fixed in the stone, were placed the rods of 
tron whose elongation he desired to measure. the vertical 
rods slid a transverse bar of brass ing “a vertical rod of 
end of the lever is connected by a very fine wire, with an axis 
on which is fixed a small circular mirror. If the steel point be 
pushed up against the agate plate, the end of the lever is raised ; 
the axis is thereby caused to turn, and the mirror rotates.” The 
angular deflections of the mirror he determined by the method 
of Pogeendorff ; that is, by viewing in a telescope the divisions 
of a fixed scale reflected from the mirror. é : 
Dr. Tyndall gives the following account of his experience 
With this apparatus. “ Biot found it impossible to work at his ex- 
periments on sound during the day in Paris; he was obliged to 
Wait for the stillness of night. I found it almost equally difficult 
make accurate experiments, requiring the telescope and scale, 
with the instrument just described, in London. Take a single 
