124 7. C. Mendenhall—Acceleration of Gravity at Tokio, Japan. 
Arr. X VII.— Determination of the Acceleration due to the Force 
of Gravity, at Tokio, Japan ; by T. C. MENDENHALL. 
THE following series of determinations of the value of the 
acceleration due to the force of gravity at this place was begun 
in the month of February of the present year and continued 
throughout the succeeding two or three months. A considera- 
ble time was devoted to preliminary experiments, and there was 
some delay on account of the non-arrival of one of the pendu- 
lums used, from the maker in Europe. Throughout the whole 
series and in all of the labor connected with it I have had the 
intelligent and faithful assistance of Messrs. Tanaka and Tan- 
akadate, two special students in the department of Physics of 
the Imperial University of Japan. 
here is probably nothing new in the method employed, 
except, perhaps, the mode of determining the time of vibration 
of the pendulum. As far as I am aware this method has not 
been previously described, and it seems to possess many advan- 
tages over the ordinary method of coincidences. It simply 
involves the use of a good chronograph and a break-circuit 
clock or chronometer, together with an arrangement by means 
of which the experimental pendulum can be made to record 
its own beats upon the chronograph at any time. In the be- 
ginning the whole number of vibrations which the pendulum 
will make in a given time may be determined by letting it 
* 
break the circuit at every vibration, or, better, at every sixtieth | 
or hundredth vibration, which can easily be accomplished by 
counting and raising the break-circuit apparatus to its proper 
position underneath the pendulum at the right moment. In 
our arrangement this apparatus consisted of a very small and 
light ‘“‘trip-hammer ” made of fine wire, which was so adjusted 
that by pressing upon a button it was brought up to such a 
point that it would be just “thrown” by the pendulum in its 
passage through the lowest point of its are. Although the 
resistance offered to the pendulum can be made extremely 
small, yet it is so great as to interfere quite perceptibly with its 
motion if the pendulum is obliged to operate the break-cireuit 
at each beat, as experiment has proved. But it may be 
rejected after the first two or three trials, not only on account 
of the resistance which it introduces but also because it is not 
necessary to continue its use. The whole number of seconds 
required for a given number of vibrations being known, it 
only remains to determine the fractional part of a second as 
accurately as possible. It is therefore only necessary to cause 
the pendulum to break the circuit twice, once at the beginning 
of the period and once again at the end, By this means all 
