MESSRS, J. MILNE AND T. GRAY ON SEISMIC EXPERIMENTS. 
869 
XVIII. and XIX.— Fall of ball. Ball fell 35 feet. 
Line A F and line A E (see sections). 
XVIII. 
XIX. 
Time of 
motion. 
Observer. 
Time of 
motion. 
Observer. 
Mean. 
Line A F, station 180 . 
Line A E, station 130 . 
seconds. 
24 
10 
Nembrini . 
Milne . . 
seconds. 
10 
13 
Milne . . 
Nembrini . 
17 
11 
These experiments apparently show that neither a small hill nor a cutting have the 
effect of preventing the propagation of vibrations. 
This brings us to the end of all the experiments where the movement in a vessel of 
mercury was used as an indicator of the relative amounts of motion which were 
transmitted to the various stations. The only value of these observations, which 
were necessarily somewhat rough, is the indication they give as to where the motion 
was relatively strong, slight, hardly perceptible, &c., and of points where it had died 
out or owing to the configuration of the ground had been unable to reach. 
Second Set oe Experiments. 
In the following three experiments, in consequence of having employed a seis¬ 
mometer, we obtained a close approximation to the maximum amplitude of the motion 
of an earth particle. These indications also showed us whether the movements 
recorded were due to normal or transverse vibrations. 
The seismometer may be briefly described as follows :— 
A frame having a spherical base supports on a pivot at a point a little above the 
centre of curvature of this base a heavy lead ring, of such a weight that when 
placed at the centre of oscillation of the frame it produces approximately neutral 
equilibrium. 
To the edge of this ring, at points distant from each other by one quarter the 
circumference, two threads were attached. These threads were carried outwards in 
the direction of the radii through their points of attachment and passed over light 
pulleys attached to a second frame which was fixed to the earth. When this second 
frame was moved, in consequence of the motion of the earth, the ring, by its inertia, 
caused the threads to turn the pulleys; these pulleys were furnished with indices 
giving a multiplication of 40 for the motion on one side of the normal position of 
the earth. 
