—_. -= = 
ON SEISMOLOGICAL INVESTIGATION. 219 
Where Mitre we is called n? so that » divided by 27 is the OATERGY 
of the natural vibration of Q. 
re @ 55 called e. 
The distance PG is called a, and GQ is 6, M is the mass of the body 
and & its radius of gyration about G. 
Assuming that friction will destroy the natural vibrations at Q, but 
neglecting the easily expressed friction term of (1), the forced vibration is 
easy to find. If an observer moves with P and A, he observes, not 2, 
but w2—2,. Let y=x—a,. Then if x,=A sin qt, 
2 
yo —h S * / Lf __ sin gt. 
y ; are is 
Now if we arrange that n is, say, less than one-fifth of ¢ [that is, that 
the natural frequency of Q is less than one-fifth of the fr equency of A and P] 
we may say that the motion y which is observed is a faithful imitation of 
any periodic motion of P and A; or, letting a+ 6 or PQ be called 7 and 
k?+a?=k,?, the square of the radius of gyration about P, y=— ke Oe 
A magnifying pointer on the spring enables this tiotion to be 
observed. 
Tt is obvious that the motion may be in a horizontal plane instead 
of a vertical. 
Note. By Professor Joun MILne. 
A form of Perry Tromometer as experimented with at Shide consists of 
a horizontal beam free to oscillate upon a knife edge. This beam is 
heavily loaded by two unequal masses which to obtain a balance are 
placed at different distances from the knife edge. Attached to one of 
these masses and running vertically upwards is a light A.P. spring, the 
top end of which is held by a fixed support. To show the movements of 
the spring which coils or uncoils with vertical vibratory motion, a very 
light pointer, or a small mirror from which a beam of light is reflected, is” 
attached to the same. One photogram representing a period of twenty- ~ 
four hours has been obtained by this instrument at Shide. This shows 
that during nearly the whole of the day the mirror is in motion, and the 
fact that this motion is due to passing carts, carriages at a distance of 
several hundred yards, and trains at a distance of about a mile speedily 
led to the conclusion that an instrument so extremely sensitive to rapid 
elastic motion could not be used at Shide. One interesting observation » 
was that, at the time of the funeral of Prince Henry of Battenberg, when — 
minute guns were being fired on ship-board at a distance of about five . 
miles, each sound wave was accompanied by the sudden displacement of 
the spot of light through a distance of about one foot. It did not séem 
that vibrations came from their origin through the ground to disturb the 
instrument, but as sound waves through the air, which shook the burlding 
and the foundation on which the instrument rested. ) 
If an instrument of this description could be installed at a locality ties 
we can assure ourselves that its movements could only be due to natural * 
