866 
MESSRS. J. MILNE AND T. GRAY ON SEISMIC EXPERIMENTS. 
First Set of Experiments. 
These were preliminary experiments to determine the relative a/mounts of energy received 
at stations variously situated with regard to the point at which the hall fell. 
The general method which, was followed in making these experiments was by means 
of a winch to wind the ball up to a certain height, pull a catch, and allow it to drop. 
To observe the amount of vibration which was produced in consequence of this blow, 
a number of similar bottles containing similar amounts of mercury were employed. 
One or two of these were placed at each station with an observer, who noted, by means 
of the seconds’ hand of a watch, the length of time that the oscillation produced by 
the blow continued. These bottles had wide necks, an internal diameter of about 
40 millims., and were filled with mercury to a depth equal to their diameter. In order 
to see the vibration of the mercury distinctly, in some of the experiments a small float 
consisting of a thin circular piece of sheet iron about 6 millims. in diameter, from the 
centre of which there was a thin piece of wire projecting up through the neck of the 
bottle, was placed upon the surface of the mercury. 
In consequence of a slight motion in the mercury producing a considerable motion 
in the ship-like mast of the float, the length of time that the mercury oscillated could 
be easily noted. These floats were made as similar as possible. After the bottle had 
been firmly planted on the ground, and the mast of the float remained steady on a 
position passing through the centre of the neck of the bottle, the whole was covered 
with a large beaker to act as a shade. 
This method of experiment is evidently at its best only capable of giving a relative 
estimate of the amount of motion at the different observing stations. 
I. —Fare of ball on the line A B. 
All the bottles, which were numbered from 1 to 6 consecutively, were placed in a 
row at the 30 feet station. The ball fell from a height of 30‘5 feet. Owing to the 
ground being very soft it sank into the soil for fully 1 foot. All the pointers oscillated 
for about 20 seconds with the exception of that in bottle number 4, which oscillated 
for nearly 30 seconds. 
II.— Fall of ball on the fine A B. Ball fell 30’5 feet. 
Number of 
Station. 
Time of motion of 
Remarks. 
bottle. 
the mercury. 
feet. 
seconds. 
6 
30 
14(?) 
These observa¬ 
41 
5 J 
\ 
100 
About 28 
tions were 
not carefully 
21 
*j| 
\ 
\ 
200 
About 15 
made. 
