DR. TYNDALL ON MOLECULAR INFLUENCES. 
219 
slider, which can be moved smoothly back and forward along a bevelled groove, by 
means of the lever L. This lever turns on a pivot near Q, and fits into a horizontal 
slit in the slider, to which it is attached by the pin p' passing through both ; in the 
lever an oblong aperture is cut through which p' passes, and in which it has a certain 
amount of lateral play, so as to enable it to push the slider forward in a straight line. 
A small chamber, m!, is cut out at the end of the slider, and across, from projection 
to projection, a thin membrane is stretched ; a chamber is thus formed bounded on 
three sides and the bottom by wood, and in front by the membrane. A thin platinum 
wire, bent up and down several times, so as to form a kind of micrometer grating, is 
laid against the back of the chamber and imbedded in the end of the slider by the 
stroke of a hammer ; the end in which the wire is imbedded is then filed down until 
about half the latter is removed, and the whole is reduced to a uniform flat surface. 
Against the common surface of the slider and wire an extremely thin plate of mica 
is glued, sufficient, simply, to interrupt all contact between the bent wire and a 
quantity of mercury which the chamber m' is destined to contain : the ends w w’ of 
the bent wire proceed to two small cisterns, c d, hollowed out in a slab of ivory ; they 
enter through the substance into the cisterns, and come thus into contact with mer- 
cury which fills the latter. The end of the slider and its bent wire are shown in 
fig. 2. The rectangular space efgh, fig. 1, is cut quite through the slab of mahogany, 
and a brass plate is screwed to the latter underneath ; from this plate (which, for 
reasons to be explained presently, is cut away as shown by the dotted lines in the 
figure) four conical ivory points, ahcd, project upwards ; though appearing to be upon 
the same plane as the upper surfaces of the bismuth and antimony bars, the points 
are in reality 0'3 of an inch below the said surfaces. 
2 G 
MDCCQLIII. 
