551 
of Edinburgh, Session 1877-78. 
Heddle thinks it probable that this fact will aid in the tracing out 
of the individual beds — far from an easy matter in that troubled 
district. 
The conclusion of the chapter is devoted to a speculation upon 
the metamorphism of these limestones. 
2. On the Strength of the Currents required to Work a 
Telephone. By Professor Tait. 
(Deferred from January 7th.) 
Perhaps the most singular fact connected with the telephone is 
the excessive feebleness of the currents which suffice to work it. I 
have had no opportunity of testing any but rough arrangements set 
up by present or former students of my own, so that I cannot judge 
how far my results may apply to the instrument as sold. 
1. A striking illustration of the feebleness of the currents re- 
quired is furnished by using a Holtz machine driven very slowly, 
without condenser, and with its terminals so close that the discharge 
is bareiy audible, and certainly invisible except in the dark. When 
insulated wires were led from these terminals to the telephone 
(placed in a distant room) the effect was very curious. The instru- 
ment gave a hissing sound, quite comparable in intensity with that 
which was produced directly when the terminals of the machine 
were widely separated, one connected with the ground and the other 
with a large conductor discharging by brushes into the air, the 
machine being turned rapidly. The telephone continued to give 
audible sounds with slow turning, even when the terminals of the 
machine (somewhat tarnished) were pressed into contact. 
2. To measure roughly the intensity of the current, I placed 
one prong of an unmagnetised tuning-fork about half an inch in 
front of the sending telephone, and measured by a microscope and 
scale the extent of its vibrations when the note just ceased to be 
audible to a listener at the receiving telephone. Next I substituted 
for the receiving telephone an exceedingly delicate astatic galvano- 
meter, with very small moment of inertia, and measured the swing 
produced by one definitely assigned motion of the prong of the 
tuning-fork. By means of a known thermo-electric couple, I deter- 
mined the strength of the current corresponding to the observed 
