ON SEISMOLOGICAL INVESTIGATIONS. a 
have overcome them. In the course of construction and testing he has 
obtained a number of interesting results of which he has made the following 
notes. It may be added that, in spite of the pressure of this work, he 
has found time to make regular visits of superintendence to Shide, either 
in company with the Chairman or alternating with him. His instru- 
mental skill and knowledge have been freely put at the disposal of the 
Committee throughout. 
Notes on the Comparison of two Milne-Shaw Seismographs. 
By J. J.S. 
The testing of two Milne-Shaw seismographs, No.8 and No. 9,at West 
Bromwich, during May of the present year, showed not only how stan- 
dardised machines may be relied upon to give similar seismograms, but 
also afforded an opportunity of investigating the questions of daily tilt 
oe microseisms, and the degree of sensitivity to which a machine can 
e set. 
Statements have been published that, due to the mechanical imperfec- 
tions of seismographs, there is a difficulty in obtaining long periods of 
oscillation ; anything of the order of 60 seconds being an impossibility. 
The facilities for making refined adjustment provided in the Milne-Shaw 
type prompted the writer to investigate the possibilities. 
No difficulty was experienced in obtaining a period of 60 seconds, and, 
in order to test its constancy, the machine was left for five days, at the 
end of which time the period remained unchanged. 
The period was then increased to 90 seconds ; the machine had now 
become highly sensitive to the slightest tilt. With a nominal magnifica- 
tion 150 times the horizontal ground movement, this period gave such a 
sensitivity to tilt that 1 sec. of are produced 1-5 metre displacement 
of the light spot ; or conversely 1 mm. displacement corresponded to a 
tilt of 1 inch in about 5,000 miles. 
A subsequent attempt produced a period of approximately 120 seconds, 
which represents a sensitivity of 1mm. amplitude for a tilt of 1 inch in 
upwards of 8,000 miles. 
With the apparatus in this condition the smallest movement of the 
observer affects the position of the light spot ; therefore the observer 
took up a seated position 6 feet from the column ; but even so, a swaying 
of the body in the chair produced an appreciable effect. 
This machine, No. 8, was mounted upon a pier measuring about a 
cubic yard, built up from the cellar floor of the writer’s house. The 
weight of one person (150 lb.) in a bedroom two floors above, and not 
immediately over the instrument, produced a tilt in the cellar floor of 
about -04 second of arc, causing the light spot to be deflected more than 
100 mm. 
A test was made of the machine’s sensitivity to temperature change 
at this 120 seconds period. Approaching the column for this purpose 
was out of the question, therefore the rays from a small Bijou incandescent 
gas mantle, with which the chamber was lighted, were, by means of a 
small hand mirror, so projected that they fell upon one side of the column 
and not upon the other. The heat from this small increase of illumination 
expanded the one side of the column sufficiently to drive the light spot 
off the scale. Ne Remand 
When timing the period of oscillation in these higher sensitivities it 
