MESSRS. J. MILNE AND T. GRAY ON SEISMIC EXPERIMENTS. 
873 
Fourth Set of Experiments. 
In these experiments a pair of horizontal lever seismographs were used to register 
the motion of the ground. They indicated their motion on smoked glass plates. 
From the following brief description of these instruments it will be seen that if one 
of them was so placed that its pointer was in the direction of the line joining the 
centre of disturbance and the observing station, it could only fully register transverse 
vibrations, whereas normal vibrations would not be recorded. 
The instruments were therefore placed with their pointers at right angles to each 
other, one of them pointing directly towards the weight. In this manner one of them 
was caused to give a full record of normal vibrations and the other of transverse 
vibrations. 
The seismograph here used was almost identical in form with Ewing’s “astatic 
horizontal lever seismograph.” 
The principle is that of a mass supported on a horizontal arm which can turn freely 
round a vertical axis. The mass consisted in this case of an iron ring about 5 kilos, 
in weight, and was pivoted so that it could turn round a vertical axis through its 
centre at right angles to the plane of the ring. The distance between the axis of the 
lever and of the mass was about 2 centims., and the length of the index such as to 
give a multiplication of 12. 
The various columns in the following records of the experiments with these instru¬ 
ments have the same meaning as those in the last set of records. 
Number 
of 
fall. 
Station. 
Maximum amplitudes. 
Remarks. 
Direct 
Transverse. 
feet. 
millims. 
millims. 
19 
100 
7-5 (?) 
1-25 
24 
50 
9 
2-5 
25 
30 
1 
On the line J K. 
Fifth Set of Experiments. 
These experiments, which were made on the line A D, chiefly differ from the third 
set in the fact that the smoked glass plates, instead of being at rest whilst the 
pointers of the seismographs were indicating the back and forth motions of the ground, 
were drawn along horizontally by means of clockwork. These plates were strips 
of glass about 2 feet in length, and from 3 to 6 inches in breadth, carried on a small 
three-wheeled carriage. This carriage was attached to a strip of paper which was 
pulled along by means of a Morse telegraph instrument. An advantage in using this 
5 T 2 
