212 On the Pendulum Experiment. 
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Anr. XVIL—The Pendulum Experiment; by Lieut. D. P. 
{i _ Woopzvry, U. 8. Corps of Engineers. 
- Wirs the exception of some tnimportant allusions to the la- 
bors or suggestions of others the following article was drawn up 
in June last. It is an attempt to present the subject in a simple 
light, and to furnish a demonstration at once rigorous and easily 
intelligible. No allusion is made to those disturbing causes which 
we can either avoid or suppose to be avoided when the experi- 
ments are made in the most perfect manner.* 
It is easy. to understand the phenomena of the Foucault prob- 
lem when the. experiment is supposed to be tried at the pole of 
the earth or on the equator, but the case becomes more complex 
when the experiment is made at any intermediate point. 
I will explain briefly the mechanical principles involved in 
this problem and then propose a new experiment analogous to 
that of the pendulum. 
Let us suppose a piece of iron, or any heavy substance, which, 
for convenience, we will call a needle, suspended, in a horizontal 
position, ina north and south line, by a thread or wire. T 
needle will continue in the meridian for any period of time, and 
in so doing its prolongation must always pass through the same 
point on the prolongation of the axis of the earth. Around that 
point in each instant of time, it describes a small angle on the 
surface of the cone. The amount of that angular velocity, as 
compared with that of the earth during the same period, is easily 
‘with the corresponding motion of the earth or 360.° In either 
way we find Bs a 
aa a Ge Tce ainiemmae., nk RR ay me " 
* For other articles in this Journal on this subjec 1 200, 
ar tee subject, see last volume, PP- 
