878 
MESSRS. J. MILNE AND T. GRAY ON SEISMIC EXPERIMENTS. 
Normal vibrations. 
Fall 38. 
These records at the 50 feet and 250 feet stations were very similar to those of 
the previous experiments. 
Transverse vibrations. 
Fall 39. 50 feet station. 
There were at the commencement 16 vibrations per second. After 1 second 
there were 8 vibrations per second. This change in the rate of vibration may 
possibly be due to a disturbance caused by the direct wave, and therefore only 
apparent. 
Vibrations continued for 2 seconds. 
250 feet station. 
There were about 8 vibrations per second. The greatest double amplitude 
is *8 millim. There is no appearance of the rapid vibrations which were observed 
at the 50 feet station. 
The characteristic waves of the 50 feet station can be here recognised. 
Transverse wave. 
Fa,11 40. 
The character of the vibrations is similar to that of the previous experiment. 
Eighth Set (Velocity). 
The method first adopted for the measurement of the velocity of transit was to 
arrange a very sensitive circuit-closer in a position only a few feet distant from the 
point where the weight fell. This circuit-closer being connected with a pair of wires 
leading to a chronograph, situated at a point 560 feet distant from the circuit-closer, 
served to bring this chronograph in action. An observer stationed beside the chrono¬ 
graph, with his hand on a contact key, broke the circuit at the instant the surface of 
mercury in a vessel placed on the ground beside him was set into vibration. The 
corrections, personal and instrumental, in this method were evidently somewhat 
difficult, and it was abandoned before any very good results were obtained by it. 
The lowest result obtained by this method was about 380 feet per second and the 
highest about 930 feet, the average being 630 feet. The explanation of this very 
high result was no doubt to be found partly in the circuit-closer, which was at that 
time in various ways imperfect, and partly in the fact that the fall of the weight could 
be readily heard through the air. This may have caused the observer to anticipate 
the very small vibration which was produced in the mercury. The circuit-closer was 
caused sometimes by the rapid up and down movement of the earth to leap out of 
contact immediately after having closed the circuit; in such cases the chronograph did 
