574 Proceedings of Royal Society of Edinburgh. [sess. 
altogether novel idea, which may possibly throw light on that 
much-debated problem — the internal constitution of the earth. 
Whatever may be the specific cause of any particular earth- 
quake, there must he at its source a disturbance or abrupt change 
in the configuration of the material of the earth’s crust. A rupture 
or dislocation occurs, and outwards, in all directions, a disturbance 
is propagated, accompanied by rupture or dislocation in nearer 
regions, by vibrations, elastic and quasi-elastic, in these and in more 
distant regions. For the study of the non-destructive vibratory 
accompaniments of earthquakes, seismologists have invented various 
forms of seismometer or seismoscope. 
The essential feature of all seismometers is the £ steady point.’ 
A mass is adjusted, by means of various devices, so as to remain 
steady although its supports move with the ground. A style or 
pen attached to this steady body is arranged so as to make a 
tracing on a surface fixed to the moving ground. 
To register small earthquakes, it is advisable to have a multi- 
plication of the relative motion of earth and steady body, so that 
the records may he studied to advantage. In Yicentini’s form of 
seismograph, the motion is magnified 100-fold by means of two 
delicately-poised levers. The record is made by a light glass fibre 
resting against a smoked surface. The same method of recording 
is adopted in other delicate Italian instruments, such as Cancani’s 
horizontal pendulums at Kocca di Papa [Rep. B.A . , 1898, p. 266). 
In his own form of horizontal pendulum, Milne uses a photo- 
graphic method of recording the motion of the end of the 
horizontal boom which hears the approximately steady body, and 
which is made to have a very long period by being attached by a 
tie to a point nearly vertically above the point on which the boom 
pivots. Darwin’s delicate bi-filar suspension and other forms of 
£ tromometer ’ and horizontal pendulum, though intended primarily 
for other purposes, may also be classified as seismometers. 
The delicacy of a seismometer varies with the function it is 
intended to perform. The instrument intended to detect and 
record minute movements which do not affect our senses must he 
much more delicate than the instrument whose function it is to 
record veritable earthquakes in the popular significance of the term. 
The greater the distance from the source at which we hope to 
