444 O. N. Rood—Application of the Horizontal Pendulum to the 
Art. XLVIIL — On the application of the Horizontal Pendulum to 
the measurement ee minute changes in the dimensions of Solid 
Bodies ; by O. N. Roop, Professor of Physics in Columbia 
ollege. 
[Read before the National Academy of Sciences, Nov. 4th, 1874.] 
In figure 1, let RR represent an inflexible rod of steel placed 
horizontally, ‘and supported at its extremities by pivots on 
which it turns freely, and let W be a weight inflextbly attached 
to the rod as indicated. It is evident that Wit when left to it- 
1, 
A now an attractive or repulsive force be 
tend to be deflected from its vertical 
position, and if the force is sufficiently 
powerful a sensible deflection will be ob- 
poeta Tn an arrangement of this kind, 
e from friction, “the opposing foree 
to be overcome will of. course be the attraction of oravita- 
tion n; if, however, we gradually elevate the rod RR, the gravity 
component will diminish, and become zero when RR i is vertical, 
and consequently AW horizontal. In his articles on the Hori- 
zontal Péadatan, Zollner* has shown experimentally how this 
may be accomplished to an almost incredible extent, so that an 
apparatus of this general nature in his hands became capable 
of obeying even the feeble attractive force of the moon! 
I give here a diagram of Zéllner’s horizontal pendulum ; at 
W is the weight, the inflexible rod being re- 
placed by fine steel wire or watch-spring, 
stretched as shown on the vertical column CO, 
ror for reflecting the divisions of a scale toa 
telescope. The height of this apparatus was 
— 30 inches; it was mounted on a pier like 
astronomical instrument, and enclosed ina 
mes building by itself, the observations being 
made from without 
n the tetirkabis papers above referred to, 
Zésllner also pointed out the extreme sensitive- 
— of this apparatus to changes in level, and it at once oc- 
urred to me that by suitable modifications and additions, it 
pa be converted into an instrument of unprecedented delicacy 
for studying minute changes in the dimensions of solid bodies. 
* Pogg. Ann. for 1873, cl, 131, 134. 
