638 
PEOFESSOE THOMSON’S NOTE ON THE FOEEGOING PAPEE. 
must produce much greater electromotive forces, which we may conceive may not unfre 
quently be as much as that of ten or twenty cells, and sometimes may amount to 100 
cells or more. Just such amounts of electromotive force were those which I actually 
observed in the Atlantic cable, as the following extract from the ‘ Encyclopaedia Metro- 
pohtana,’ article “ Telegraph, electric,” shows. 
“ In the failure of the Atlantic Cable in September 1858, the portion terminating at 
V alencia came to give nearly the same indications as an insulated conductor about 270 miles 
long, laid out westward, and connected with a copper plate sunk at a little less than that 
distance in the Atlantic. In these circumstances the writer found that from 1 to 9 or 10 
twentieths of the electromotive force of two Daniell’s elements was generally sufi&cient 
to balance the earth-current; not unfrequently 14 or 15 were required; sometimes, 
although rarely, 20, or the full electromotive force of two Daniell’s elements, was 
insufficient ; and once or twice in the course of the month of September, earth-currents 
were received so strong that five or six Daniell’s elements would have been required to 
balance them.” 
It seems therefore quite certain that the ordinary every-day earth-currents in that 
locality must be very sensibly influenced by electro-magnetic induction from the ordinary 
diurnal variations of terrestrial magnetism ; but it is also quite certain that they are only 
in part due to this cause, and that some more powerful, but as yet unknown agency, is 
at work to produce them. For although I found that a day seldom, if ever, passes 
without the direction of the current changing several times, yet there was no relation 
between the times of such changes and the solar hours. I conclude with the following 
additional extract from the same article, expressing views regarding earth-currents which 
I think will be found to agree with the extensive and careful observations of Mr. C. V. 
Walkee, which have been published since it was written, although they seem quite at 
variance with the theory which has recently been advocated by Prof. Lamont and Dr. 
Lloyd, that earth-currents, however they are themselves generated, do directly produce 
the magnetic variations. 
“ Earth-currents are certainly related to the irregular variations of terrestrial magnetism, 
since they are always found unusually strong during brilliant displays of aurora borealis ; 
for it has long been known that, on these occasions, the magnetic disturbances are un- 
usually strong. Being related to the variations of terrestrial magnetism, it is probable 
that the earth-currents also will be found to have daily periods ; but, in the mean time, 
we only know that, while the diurnal variation in terrestrial magnetism is observable in 
general every day, and is only on rare occasions overborne by irregular disturbances, the 
earth-currents vary each day from hour to hour, like the wind, under some overpowering 
non-periodic influence, and can only show daily periodicity in residual averages derived 
from lengthened series of observations. It is probable that careful synchronous obser- 
vations of auroras, earth-currents, and variations of terrestrial magnetism, will lead to a 
discovery of the primary influence, whether in the earth, or terrestrial atmosphere, 
or surrounding interplanetary air, which causes these phenomena.” — W. T. 
