6 REPORTS ON THE STATE OF SCIENCE.—1917. 
was noted that the effect of amplitude upon the period became very 
marked—increasing the amplitude of swing rapidly increased the period. 
With the pendulum set to oscillate in 10 or 12 seconds this difference 
amounts to only about 1); of a second over a wide range of amplitude ; 
but at 120 seconds the fluctuation becomes important. With a change 
in amplitude from 1,000mm. to 100mm. there was a drop in the period 
of 20 to 30 per cent. ‘ 
Time did not permit of an investigation to determine the rate of change 
with differing periods. There was insufficient change in the damping 
ratio to account for the phenomenon, therefore it is probable that it is an 
extreme case of ‘ circular error.’ 
This variation of period with amplitude, even when small, suggests 
that some standard amplitude should be used when determining the 
period. 10 mm. is the prescribed standard with the Milne-Shaw machine. 
The Milne-Shaw boom is short, and the magnification includes 
mechanical leverage. Though the friction of same is extremely small. 
it was expected that it would be sufficient to operate against obtaining 
excessively long periods, and would compare unfavourably with a simple 
elongated pendulum of the Milne type. The result was quite the reverse, 
thus establishing the fact that the air resistance on a long boom forms the 
major part of the total friction, and suggests that, though the design is 
simple, it is not necessarily the best for obtaining free oscillations. 
The second machine, No. 9, was mounted upon a pier in an out-building 
60 feet from No. 8. It was oriented in the same azimuth, and connected 
in series with the same time circuit. The constants of both machines 
were made equal, viz. :-— 
Period Supe hl SK Ae seconds: 
Sensitivity to tilt . . 26 mm. = I sec. of are. 
Magnification . . « 15031. 
Dampingwratio.. |. . 20s Ns 
The early part of the month was favoured by calm nights, and it was at 
once observed that the microseisms were identical on both machines, 
in epoch, phase, period, and amplitude, thus demonstrating that micro- 
seisms are pure ground movements as distinguished from convection 
currents in the observatory or instrument cases, and that the term 
‘ air-tremors,’ as used by so many observers, is a misnomer ; for it is not 
conceivable that air disturbances should so exactly coincide in separate 
buildings 60 feet apart. It was found that when the microseisms were 
intermittent they could easily be identified by their amplitude and number 
of waves in a group, also by the interval between successive groups (see 
A and B, Plate). In the past, microseisms have been investigated by 
observing their period and intensity, and the results compared with 
similar data from other stations. It is here suggested that a more fruitful 
method may be by gradually separating two or more machines, com- 
mencing with a few hundred yards or a mile or two, and, if the trains of 
waves could be still identified, increasing the distance to form a base 
line of sufficient length to determine their speed of propagation and the 
direction in which they were travelling. If this much could be achieved 
it is possible their cause and origin might be discovered. 
A horizontal pendulum has two types of sensitivity, one to tilt, and the 
other to horizontal displacement. The former is regulated by the inclina- 
tion of its supports ; the latter is a result of its design, and is proportional 
