882 
MESSRS. J. MILNE AND T. GRAY ON SEISMIC EXPERIMENTS. 
Fig 8. (See seventh set of experiments, fall 37 or 38). This diagram is a magnified 
representation of two records of normal vibrations taken simultaneously, 
the upper one at a 50 feet station and the lower one at a 250 feet 
station on the same line. Corresponding points in time on these two 
diagrams are marked with similar figures. 
The chief point illustrated is the last and probably the best method of 
obtaining velocity. 
By comparing the two records we see that vibrations took place at the 
50 feet station considerably before reaching the 250 feet station. 
By measuring the interval between similar points, as for instance the 
beginning of the motion on the two diagrams, the time taken for the 
vibration to pass from one station to the other can be calculated, and 
from this the velocity of propagation between these two stations deduced. 
This method of experiment has the great advantage that no instrumental 
error can enter into the result. It is of course possible that a vibration 
which reached the 50 foot station, and was there registered as the 
beginning of the motion, might not reach the 250 foot station with 
sufficient amplitude to be registered. An error from this cause can, 
however, be easily avoided by an examination of the records. 
General Conclusions. 
The first set of experiments, which was an attempt to obtain an estimate of the rate 
at which a disturbance produced at a, point on the earths surface is absorbed, were 
not very satisfactory. The times which the mercury continued to vibrate at the 
various stations, as given by various observers, did not agree well with each other, and 
no doubt depended to a great extent on the mode of observing. When, for instance, 
the reflection of the sun from the surface of the mercury could be seen, the motion 
could be detected much longer than when a diffused light was used. 
The observations all show a dying out of the vibration, as a matter of course, but 
the only interesting observation is the total cutting off of the vibration by the pond 
when the point of observation was at a sufficient distance from the corner. This 
shows that the motion observed near the corner must have been due to the creeping 
round of the vibrations transmitted along the side. More definite information is given 
by the second set of experiments. These show that the vibrations which pass up the 
hill and round the pond are for the most part transverse. 
In the third set of experiments we have evidence from the written records that the 
amplitude of motion is nearly inversely as the distance. The change in the nature of 
the ground under the falling weight interfered considerably, however, in these experi¬ 
ments. The character of these static records is also very interesting, as this no doubt 
