880 
MESSRS. J, MILNE AND T. GRAY ON SEISMIC EXPERIMENTS. 
been referred to, and for a full account of them see Transactions of the Seismological 
Society of Japan, vol. ii. 
The measurements made by the first method here mentioned gave a velocity of 
transit equal to 446 feet per second for direct and 353 for transverse wave. 
The individual experiments in this case give for direct wave 454, 446, 436, 449, for 
transverse 360, 345. 
By the second method, which evidently requires no correction of any kind except 
for clock rate, we obtained a mean velocity of 396*5 feet for direct wave. 
The individual experiments being 399, 394, the mean velocity for transverse wave 
was at the same time found to be 360 from two experiments giving respectively 367 
and 353, the rate at which the plate moved was determined by causing a small pen¬ 
dulum to act as a periodic circuit-closer, and by so doing to make a series of marks 
simultaneously on the two plates at distances apart which represented in the actual 
experiments ^§ths of a second of time. 
The variations in velocity, although partly due to different methods of experi¬ 
menting, were no doubt to a certain extent due to different conditions of the soil, 
there being a variable amount of frost during the period (somewhat extended) which 
we found necessary for the whole series of experiments. 
The difference between the results of the last two methods are probably due to 
difference in the ground. An interval of more than a week intervened between these 
two set of determinations, and in the meantime the weather had changed from, a 
minimum temperature a few degrees below zero to a minimum temperature a little 
above zero. 
Giving double value to the last two sets of experiments and leaving out altogether 
the first set we obtain a mean velocity for the direct wave of 438 feet. Again, from 
the last two sets we get a mean of 357 for velocity of transverse wave. These 
results are probably near the truth. 
Description op Figures. 
PLATE 52. 
Fig. 4. (See fifth set of experiments, fall 25). This shows a representation, magnified 
two and a-half times, of the diagram drawn at the 250 feet station by the 
rolling seismograph. 
The arrow crossing the diagram shows the direction of the direct wave. 
The first two vibrations are apparently normal ones, but the succeeding 
waves show the interference of these vibrations with transverse motions. 
