870 
MESSRS. J. MILNE AND T. GRAY ON SEISMIC EXPERIMENTS. 
Experiment XX.—Ball fell 85 feet. 
Line A E. Station 130 feet. 
The seismometer was so placed that the thread of one pointer bore about 
E. and W., or was parallel to the edge of the pond, whilst the thread of the 
second pointer was at right angles to the first thread. 
The second pointer moved about I millim. as if by a transverse wave. 
This shows a total motion of *05 millim. in the ground. 
Experiment XXI.—Ball fell 35 feet. 
Line A E. Station 130 feet. 
The seismometer was so placed that one thread pointed directly to the point 
where the ball fell. At the time of the shock the pointer of this thread did not 
move, but the pointer at right angles moved about 1 miJlim., again indicating 
that the instrument had only been affected by transversal vibrations. 
Experiment XXII.—Ball fell 35 feet. 
Line AH. 100 feet station. 
The seismometer was so placed that the thread of one pointer was in the 
direction of the weight, the thread of the other pointer being at right angles 
to this direction. 
Each of the pointers moved about 2 millims. 
Third Set of Experiments. 
In the following experiments a clear graphical distinction between normal and 
transverse vibrations was obtained at a number of different stations, together with 
the maximum amplitudes of each of these two distinct movements. 
The instrument employed in obtaining these records was a small rolling sphere 
seismograph, writing its movements directly upon a smoked glass plate by means of a 
pointer. 
The instrument may be described as follows:—At the centre of curvature of a 
small hemisphere a heavy lead ring rested on a point. To the hemisphere a light 
pointer was attached, of such a length that its end moved ten times as far as the 
bottom of the hemisphere. At the end of this pointer a sliding needle was arranged 
so that its point rested against a smoked glass plate, and when moved by the motion 
of the earth under the sphere, wrote that motion magnified 10 times on the plate. 
When the ball fell, the pointer could be seen to make one or more back and forth 
quick movements in the direction of the line joining the instrument and the point 
where the ball fell, and then immediately afterwards to change suddenly into a motion 
which was inclined to the first movements. The records drawn upon the smoked 
glass plates corresponded with the observed movements. 
